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DO NOT READ THIS FILE ON GITHUB, GUIDES ARE PUBLISHED ON https://guides.rubyonrails.org.

Configuring Rails Applications

This guide covers the configuration and initialization features available to Rails applications.

After reading this guide, you will know:

Locations for Initialization Code

Rails offers four standard spots to place initialization code:

Running Code Before Rails

In the rare event that your application needs to run some code before Rails itself is loaded, put it above the call to require "rails/all" in config/application.rb.

Configuring Rails Components

In general, the work of configuring Rails means configuring the components of Rails, as well as configuring Rails itself. The configuration file config/application.rb and environment-specific configuration files (such as config/environments/production.rb) allow you to specify the various settings that you want to pass down to all of the components.

For example, you could add this setting to config/application.rb file:

config.time_zone = "Central Time (US & Canada)"

This is a setting for Rails itself. If you want to pass settings to individual Rails components, you can do so via the same config object in config/application.rb:

config.active_record.schema_format = :ruby

Rails will use that particular setting to configure Active Record.

WARNING: Use the public configuration methods over calling directly to the associated class. e.g. Rails.application.config.action_mailer.options instead of ActionMailer::Base.options.

NOTE: If you need to apply configuration directly to a class, use a lazy load hook in an initializer to avoid autoloading the class before initialization has completed. This will break because autoloading during initialization cannot be safely repeated when the app reloads.

Versioned Default Values

config.load_defaults loads default configuration values for a target version and all versions prior. For example, config.load_defaults 6.1 will load defaults for all versions up to and including version 6.1.

Below are the default values associated with each target version. In cases of conflicting values, newer versions take precedence over older versions.

Default Values for Target Version 8.0

Default Values for Target Version 7.2

Default Values for Target Version 7.1

Default Values for Target Version 7.0

Default Values for Target Version 6.1

Default Values for Target Version 6.0

Default Values for Target Version 5.2

Default Values for Target Version 5.1

Default Values for Target Version 5.0

Rails General Configuration

The following configuration methods are to be called on a ::Rails::Railtie object, such as a subclass of ::Rails::Engine or ::Rails::Application.

config.add_autoload_paths_to_load_path

Says whether autoload paths have to be added to $LOAD_PATH. It is recommended to be set to false in :zeitwerk mode early, in config/application.rb. Zeitwerk uses absolute paths internally, and applications running in :zeitwerk mode do not need require_dependency, so models, controllers, jobs, etc. do not need to be in $LOAD_PATH. Setting this to false saves Ruby from checking these directories when resolving require calls with relative paths, and saves Bootsnap work and RAM, since it does not need to build an index for them.

The default value depends on the config.load_defaults target version:

Starting with version The default value is
(original) true
7.1 false

The lib directory is not affected by this flag, it is added to $LOAD_PATH always.

config.after_initialize

Takes a block which will be run after Rails has finished initializing the application. That includes the initialization of the framework itself, engines, and all the application's initializers in config/initializers. Note that this block will be run for rake tasks. Useful for configuring values set up by other initializers:

config.after_initialize do
  ActionView::Base.sanitized_allowed_tags.delete "div"
end

config.after_routes_loaded

Takes a block which will be run after Rails has finished loading the application routes. This block will also be run whenever routes are reloaded.

config.after_routes_loaded do
  # Code that does something with Rails.application.routes
end

config.allow_concurrency

Controls whether requests should be handled concurrently. This should only be set to false if application code is not thread safe. Defaults to true.

config.asset_host

Sets the host for the assets. Useful when CDNs are used for hosting assets, or when you want to work around the concurrency constraints built-in in browsers using different domain aliases. Shorter version of config.action_controller.asset_host.

config.assume_ssl

Makes application believe that all requests are arriving over SSL. This is useful when proxying through a load balancer that terminates SSL, the forwarded request will appear as though it's HTTP instead of HTTPS to the application. This makes redirects and cookie security target HTTP instead of HTTPS. This middleware makes the server assume that the proxy already terminated SSL, and that the request really is HTTPS.

config.autoflush_log

Enables writing log file output immediately instead of buffering. Defaults to true.

config.autoload_lib(ignore:)

This method adds lib to config.autoload_paths and config.eager_load_paths.

Normally, the lib directory has subdirectories that should not be autoloaded or eager loaded. Please, pass their name relative to lib in the required ignore keyword argument. For example,

config.autoload_lib(ignore: %w(assets tasks generators))

Please, see more details in the autoloading guide.

config.autoload_lib_once(ignore:)

The method config.autoload_lib_once is similar to config.autoload_lib, except that it adds lib to config.autoload_once_paths instead.

By calling config.autoload_lib_once, classes and modules in lib can be autoloaded, even from application initializers, but won't be reloaded.

config.autoload_once_paths

Accepts an array of paths from which Rails will autoload constants that won't be wiped per request. Relevant if reloading is enabled, which it is by default in the development environment. Otherwise, all autoloading happens only once. All elements of this array must also be in autoload_paths. Default is an empty array.

config.autoload_paths

Accepts an array of paths from which Rails will autoload constants. Default is an empty array. Since Rails 6, it is not recommended to adjust this. See Autoloading and Reloading Constants.

config.beginning_of_week

Sets the default beginning of week for the application. Accepts a valid day of week as a symbol (e.g. :monday).

config.cache_classes

Old setting equivalent to !config.enable_reloading. Supported for backwards compatibility.

config.cache_store

Configures which cache store to use for Rails caching. Options include one of the symbols :memory_store, :file_store, :mem_cache_store, :null_store, :redis_cache_store, or an object that implements the cache API. Defaults to :file_store. See Cache Stores for per-store configuration options.

config.colorize_logging

Specifies whether or not to use ANSI color codes when logging information. Defaults to true.

config.consider_all_requests_local

Is a flag. If true then any error will cause detailed debugging information to be dumped in the HTTP response, and the ::Rails::Info controller will show the application runtime context in /rails/info/properties. true by default in the development and test environments, and false in production. For finer-grained control, set this to false and implement show_detailed_exceptions? in controllers to specify which requests should provide debugging information on errors.

config.console

Allows you to set the class that will be used as console when you run bin/rails console. It's best to run it in the console block:

console do
  # this block is called only when running console,
  # so we can safely require pry here
  require "pry"
  config.console = Pry
end

config.content_security_policy_nonce_directives

See Adding a Nonce in the Security Guide

config.content_security_policy_nonce_generator

See Adding a Nonce in the Security Guide

config.content_security_policy_report_only

See Reporting Violations in the Security Guide

config.credentials.content_path

The path of the encrypted credentials file.

Defaults to config/credentials/#{Rails.env}.yml.enc if it exists, or config/credentials.yml.enc otherwise.

NOTE: In order for the bin/rails credentials commands to recognize this value, it must be set in config/application.rb or config/environments/#{Rails.env}.rb.

config.credentials.key_path

The path of the encrypted credentials key file.

Defaults to config/credentials/#{Rails.env}.key if it exists, or config/master.key otherwise.

NOTE: In order for the bin/rails credentials commands to recognize this value, it must be set in config/application.rb or config/environments/#{Rails.env}.rb.

config.debug_exception_response_format

Sets the format used in responses when errors occur in the development environment. Defaults to :api for API only apps and :default for normal apps.

config.disable_sandbox

Controls whether or not someone can start a console in sandbox mode. This is helpful to avoid a long running session of sandbox console, that could lead a database server to run out of memory. Defaults to false.

config.dom_testing_default_html_version

Controls whether an HTML4 parser or an HTML5 parser is used by default by the test helpers in Action View, Action Dispatch, and rails-dom-testing.

The default value depends on the config.load_defaults target version:

Starting with version The default value is
(original) :html4
7.1 :html5 (see NOTE)

NOTE: Nokogiri's HTML5 parser is not supported on JRuby, so on JRuby platforms Rails will fall back to :html4.

config.eager_load

When true, eager loads all registered config.eager_load_namespaces. This includes your application, engines, Rails frameworks, and any other registered namespace.

config.eager_load_namespaces

Registers namespaces that are eager loaded when config.eager_load is set to true. All namespaces in the list must respond to the eager_load! method.

config.eager_load_paths

Accepts an array of paths from which Rails will eager load on boot if config.eager_load is true. Defaults to every folder in the app directory of the application.

config.enable_reloading

If config.enable_reloading is true, application classes and modules are reloaded in between web requests if they change. Defaults to true in the development environment, and false in the production environment.

The predicate config.reloading_enabled? is also defined.

config.encoding

Sets up the application-wide encoding. Defaults to UTF-8.

config.exceptions_app

Sets the exceptions application invoked by the ShowException middleware when an exception happens. Defaults to ActionDispatch::PublicExceptions.new(Rails.public_path).

Exceptions applications need to handle ::ActionDispatch::Http::MimeNegotiation::InvalidType errors, which are raised when a client sends an invalid Accept or Content-Type header. The default ::ActionDispatch::PublicExceptions application does this automatically, setting Content-Type to text/html and returning a 406 Not Acceptable status. Failure to handle this error will result in a 500 Internal Server Error.

Using the Rails.application.routes RouteSet as the exceptions application also requires this special handling. It might look something like this:

# config/application.rb
config.exceptions_app = CustomExceptionsAppWrapper.new(exceptions_app: routes)

# lib/custom_exceptions_app_wrapper.rb
class CustomExceptionsAppWrapper
  def initialize(exceptions_app:)
    @exceptions_app = exceptions_app
  end

  def call(env)
    request = ActionDispatch::Request.new(env)

    fallback_to_html_format_if_invalid_mime_type(request)

    @exceptions_app.call(env)
  end

  private
    def fallback_to_html_format_if_invalid_mime_type(request)
      request.formats
    rescue ActionDispatch::Http::MimeNegotiation::InvalidType
      request.set_header "CONTENT_TYPE", "text/html"
    end
end

config.file_watcher

Is the class used to detect file updates in the file system when config.reload_classes_only_on_change is true. Rails ships with ::ActiveSupport::FileUpdateChecker, the default, and ::ActiveSupport::EventedFileUpdateChecker. Custom classes must conform to the ::ActiveSupport::FileUpdateChecker API.

Using ::ActiveSupport::EventedFileUpdateChecker depends on the listen gem:

group :development do
  gem "listen", "~> 3.5"
end

On Linux and macOS no additional gems are needed, but some are required for *BSD and for Windows.

Note that some setups are unsupported.

config.filter_parameters

Used for filtering out the parameters that you don't want shown in the logs, such as passwords or credit card numbers. It also filters out sensitive values of database columns when calling #inspect on an Active Record object. By default, Rails filters out passwords by adding the following filters in config/initializers/filter_parameter_logging.rb.

Rails.application.config.filter_parameters += [
  :passw, :email, :secret, :token, :_key, :crypt, :salt, :certificate, :otp, :ssn, :cvv, :cvc
]

Parameters filter works by partial matching regular expression.

config.filter_redirect

Used for filtering out redirect urls from application logs.

Rails.application.config.filter_redirect += ["s3.amazonaws.com", /private-match/]

The redirect filter works by testing that urls include strings or match regular expressions.

config.force_ssl

Forces all requests to be served over HTTPS, and sets "https://" as the default protocol when generating URLs. Enforcement of HTTPS is handled by the ::ActionDispatch::SSL middleware, which can be configured via config.ssl_options.

config.helpers_paths

Defines an array of additional paths to load view helpers.

config.host_authorization

Accepts a hash of options to configure the HostAuthorization middleware

config.hosts

An array of strings, regular expressions, or IPAddr used to validate the Host header. Used by the HostAuthorization middleware to help prevent DNS rebinding attacks.

config.javascript_path

Sets the path where your app's JavaScript lives relative to the app directory and the default value is javascript. An app's configured javascript_path will be excluded from autoload_paths.

config.log_file_size

Defines the maximum size of the Rails log file in bytes. Defaults to 104_857_600 (100 MiB) in development and test, and unlimited in all other environments.

config.log_formatter

Defines the formatter of the Rails logger. This option defaults to an instance of ::ActiveSupport::Logger::SimpleFormatter for all environments. If you are setting a value for config.logger you must manually pass the value of your formatter to your logger before it is wrapped in an ::ActiveSupport::TaggedLogging instance, Rails will not do it for you.

config.log_level

Defines the verbosity of the Rails logger. This option defaults to :debug for all environments except production, where it defaults to :info. The available log levels are: :debug, :info, :warn, :error, :fatal, and :unknown.

config.log_tags

Accepts a list of methods that the request object responds to, a Proc that accepts the request object, or something that responds to to_s. This makes it easy to tag log lines with debug information like subdomain and request id - both very helpful in debugging multi-user production applications.

config.logger

Is the logger that will be used for Rails.logger and any related Rails logging such as ActiveRecord::Base.logger. It defaults to an instance of ::ActiveSupport::TaggedLogging that wraps an instance of ::ActiveSupport::Logger which outputs a log to the log/ directory. You can supply a custom logger, to get full compatibility you must follow these guidelines:

class MyLogger < ::Logger
  include ActiveSupport::LoggerSilence
end

mylogger           = MyLogger.new(STDOUT)
mylogger.formatter = config.log_formatter
config.logger      = ActiveSupport::TaggedLogging.new(mylogger)

config.middleware

Allows you to configure the application's middleware. This is covered in depth in the Configuring Middleware section below.

config.precompile_filter_parameters

When true, will precompile config.filter_parameters using ActiveSupport::ParameterFilter.precompile_filters.

The default value depends on the config.load_defaults target version:

Starting with version The default value is
(original) false
7.1 true

config.public_file_server.enabled

Configures whether Rails should serve static files from the public directory. Defaults to true.

If the server software (e.g. NGINX or Apache) should serve static files instead, set this value to false.

config.railties_order

Allows manually specifying the order that Railties/Engines are loaded. The default value is [:all].

config.railties_order = [Blog::Engine, :main_app, :all]

config.rake_eager_load

When true, eager load the application when running Rake tasks. Defaults to false.

config.relative_url_root

Can be used to tell Rails that you are deploying to a subdirectory. The default is ENV['RAILS_RELATIVE_URL_ROOT'].

config.reload_classes_only_on_change

Enables or disables reloading of classes only when tracked files change. By default tracks everything on autoload paths and is set to true. If config.enable_reloading is false, this option is ignored.

config.require_master_key

Causes the app to not boot if a master key hasn't been made available through ENV["RAILS_MASTER_KEY"] or the config/master.key file.

config.sandbox_by_default

When true, rails console starts in sandbox mode. To start rails console in non-sandbox mode, --no-sandbox must be specified. This is helpful to avoid accidental writing to the production database. Defaults to false.

config.secret_key_base

The fallback for specifying the input secret for an application's key generator. It is recommended to leave this unset, and instead to specify a secret_key_base in config/credentials.yml.enc. See the secret_key_base API documentation for more information and alternative configuration methods.

config.server_timing

When true, adds the ServerTiming middleware to the middleware stack. Defaults to false, but is set to true in the default generated config/environments/development.rb file.

config.session_options

Additional options passed to config.session_store. You should use config.session_store to set this instead of modifying it yourself.

config.session_store :cookie_store, key: "_your_app_session"
config.session_options # => {key: "_your_app_session"}

config.session_store

Specifies what class to use to store the session. Possible values are :cache_store, :cookie_store, :mem_cache_store, a custom store, or :disabled. :disabled tells Rails not to deal with sessions.

This setting is configured via a regular method call, rather than a setter. This allows additional options to be passed:

config.session_store :cookie_store, key: "_your_app_session"

If a custom store is specified as a symbol, it will be resolved to the ::ActionDispatch::Session namespace:

# use ActionDispatch::Session::MyCustomStore as the session store
config.session_store :my_custom_store

The default store is a cookie store with the application name as the session key.

config.silence_healthcheck_path

Specifies the path of the health check that should be silenced in the logs. Uses ::Rails::Rack::SilenceRequest to implement the silencing. All in service of keeping health checks from clogging the production logs, especially for early-stage applications.

config.silence_healthcheck_path = "/up"

config.ssl_options

Configuration options for the ::ActionDispatch::SSL middleware.

The default value depends on the config.load_defaults target version:

Starting with version The default value is
(original) {}
5.0 { hsts: { subdomains: true } }

config.time_zone

Sets the default time zone for the application and enables time zone awareness for Active Record.

config.x

Used to easily add nested custom configuration to the application config object

config.x.payment_processing.schedule = :daily
Rails.configuration.x.payment_processing.schedule # => :daily

See Custom Configuration

config.yjit

Enables YJIT as of Ruby 3.3, to bring sizeable performance improvements. If you are deploying to a memory constrained environment you may want to set this to false.

Starting with version The default value is
(original) false
7.2 true

Configuring Assets

config.assets.css_compressor

Defines the CSS compressor to use. It is set by default by sass-rails. The unique alternative value at the moment is :yui, which uses the yui-compressor gem.

config.assets.js_compressor

Defines the JavaScript compressor to use. Possible values are :terser, :closure, :uglifier, and :yui, which require the use of the terser, closure-compiler, uglifier, or yui-compressor gems respectively.

config.assets.gzip

A flag that enables the creation of gzipped version of compiled assets, along with non-gzipped assets. Set to true by default.

config.assets.paths

Contains the paths which are used to look for assets. Appending paths to this configuration option will cause those paths to be used in the search for assets.

config.assets.precompile

Allows you to specify additional assets (other than application.css and application.js) which are to be precompiled when bin/rails assets:precompile is run.

config.assets.unknown_asset_fallback

Allows you to modify the behavior of the asset pipeline when an asset is not in the pipeline, if you use sprockets-rails 3.2.0 or newer.

The default value depends on the config.load_defaults target version:

Starting with version The default value is
(original) true
5.1 false

config.assets.prefix

Defines the prefix where assets are served from. Defaults to /assets.

config.assets.manifest

Defines the full path to be used for the asset precompiler's manifest file. Defaults to a file named manifest-<random>.json in the config.assets.prefix directory within the public folder.

config.assets.digest

Enables the use of SHA256 fingerprints in asset names. Set to true by default.

config.assets.debug

Disables the concatenation and compression of assets. Set to true by default in development.rb.

config.assets.version

Is an option string that is used in SHA256 hash generation. This can be changed to force all files to be recompiled.

config.assets.compile

Is a boolean that can be used to turn on live Sprockets compilation in production.

config.assets.logger

Accepts a logger conforming to the interface of Log4r or the default Ruby Logger class. Defaults to the same configured at config.logger. Setting config.assets.logger to false will turn off served assets logging.

config.assets.quiet

Disables logging of assets requests. Set to true by default in config/environments/development.rb.

Configuring Generators

Rails allows you to alter what generators are used with the config.generators method. This method takes a block:

config.generators do |g|
  g.orm :active_record
  g.test_framework :test_unit
end

The full set of methods that can be used in this block are as follows:

Configuring Middleware

Every Rails application comes with a standard set of middleware which it uses in this order in the development environment:

::ActionDispatch::HostAuthorization

Prevents against DNS rebinding and other Host header attacks. It is included in the development environment by default with the following configuration:

Rails.application.config.hosts = [
  IPAddr.new("0.0.0.0/0"),        # All IPv4 addresses.
  IPAddr.new("::/0"),             # All IPv6 addresses.
  "localhost",                    # The localhost reserved domain.
  ENV["RAILS_DEVELOPMENT_HOSTS"]  # Additional comma-separated hosts for development.
]

In other environments Rails.application.config.hosts is empty and no Host header checks will be done. If you want to guard against header attacks on production, you have to manually permit the allowed hosts with:

Rails.application.config.hosts << "product.com"

The host of a request is checked against the hosts entries with the case operator (#===), which lets hosts support entries of type Regexp, Proc and IPAddr to name a few. Here is an example with a regexp.

# Allow requests from subdomains like `www.product.com` and
# `beta1.product.com`.
Rails.application.config.hosts << /.*\.product\.com/

The provided regexp will be wrapped with both anchors (\A and \z) so it must match the entire hostname. /product.com/, for example, once anchored, would fail to match www.product.com.

A special case is supported that allows you to permit all sub-domains:

# Allow requests from subdomains like `www.product.com` and
# `beta1.product.com`.
Rails.application.config.hosts << ".product.com"

You can exclude certain requests from Host Authorization checks by setting config.host_authorization.exclude:

# Exclude requests for the /healthcheck/ path from host checking
Rails.application.config.host_authorization = {
  exclude: ->(request) { request.path.include?("healthcheck") }
}

When a request comes to an unauthorized host, a default Rack application will run and respond with 403 Forbidden. This can be customized by setting config.host_authorization.response_app. For example:

Rails.application.config.host_authorization = {
  response_app: -> env do
    [400, { "Content-Type" => "text/plain" }, ["Bad Request"]]
  end
}

::ActionDispatch::ServerTiming

Adds the Server-Timing header to the response, which includes performance metrics from the server. This data can be viewed by inspecting the response in the Network panel of the browser's Developer Tools. Most browsers provide a Timing tab that visualizes the data.

::ActionDispatch::SSL

Forces every request to be served using HTTPS. Enabled if config.force_ssl is set to true. Options passed to this can be configured by setting config.ssl_options.

::ActionDispatch::Static

Is used to serve static assets. Disabled if config.public_file_server.enabled is false. Set config.public_file_server.index_name if you need to serve a static directory index file that is not named index. For example, to serve main.html instead of index.html for directory requests, set config.public_file_server.index_name to "main".

::ActionDispatch::Executor

Allows thread safe code reloading. Disabled if config.allow_concurrency is false, which causes Rack::Lock to be loaded. Rack::Lock wraps the app in mutex so it can only be called by a single thread at a time.

::ActiveSupport::Cache::Strategy::LocalCache

Serves as a basic memory backed cache. This cache is not thread safe and is intended only for serving as a temporary memory cache for a single thread.

Rack::Runtime

Sets an X-Runtime header, containing the time (in seconds) taken to execute the request.

::Rails::Rack::Logger

Notifies the logs that the request has begun. After request is complete, flushes all the logs.

::ActionDispatch::ShowExceptions

Rescues any exception returned by the application and renders nice exception pages if the request is local or if config.consider_all_requests_local is set to true. If config.action_dispatch.show_exceptions is set to :none, exceptions will be raised regardless.

::ActionDispatch::RequestId

Makes a unique X-Request-Id header available to the response and enables the ActionDispatch::Request#uuid method. Configurable with config.action_dispatch.request_id_header.

::ActionDispatch::RemoteIp

Checks for IP spoofing attacks and gets valid client_ip from request headers. Configurable with the config.action_dispatch.ip_spoofing_check, and config.action_dispatch.trusted_proxies options.

Rack::Sendfile

Intercepts responses whose body is being served from a file and replaces it with a server specific X-Sendfile header. Configurable with config.action_dispatch.x_sendfile_header.

::ActionDispatch::Callbacks

Runs the prepare callbacks before serving the request.

::ActionDispatch::Cookies

Sets cookies for the request.

::ActionDispatch::Session::CookieStore

Is responsible for storing the session in cookies. An alternate middleware can be used for this by changing config.session_store.

::ActionDispatch::Flash

Sets up the flash keys. Only available if config.session_store is set to a value.

Rack::MethodOverride

Allows the method to be overridden if params[:_method] is set. This is the middleware which supports the PATCH, PUT, and DELETE HTTP method types.

Rack::Head

Returns an empty body for all HEAD requests. It leaves all other requests unchanged.

Adding Custom Middleware

Besides these usual middleware, you can add your own by using the config.middleware.use method:

config.middleware.use Magical::Unicorns

This will put the Magical::Unicorns middleware on the end of the stack. You can use insert_before if you wish to add a middleware before another.

config.middleware.insert_before Rack::Head, Magical::Unicorns

Or you can insert a middleware to exact position by using indexes. For example, if you want to insert Magical::Unicorns middleware on top of the stack, you can do it, like so:

config.middleware.insert_before 0, Magical::Unicorns

There's also insert_after which will insert a middleware after another:

config.middleware.insert_after Rack::Head, Magical::Unicorns

Middlewares can also be completely swapped out and replaced with others:

config.middleware.swap ActionController::Failsafe, Lifo::Failsafe

Middlewares can be moved from one place to another:

config.middleware.move_before ActionDispatch::Flash, Magical::Unicorns

This will move the Magical::Unicorns middleware before ::ActionDispatch::Flash. You can also move it after:

config.middleware.move_after ActionDispatch::Flash, Magical::Unicorns

They can also be removed from the stack completely:

config.middleware.delete Rack::MethodOverride

Configuring i18n

All these configuration options are delegated to the I18n library.

config.i18n.available_locales

Defines the permitted available locales for the app. Defaults to all locale keys found in locale files, usually only :en on a new application.

config.i18n.default_locale

Sets the default locale of an application used for i18n. Defaults to :en.

config.i18n.enforce_available_locales

Ensures that all locales passed through i18n must be declared in the available_locales list, raising an I18n::InvalidLocale exception when setting an unavailable locale. Defaults to true. It is recommended not to disable this option unless strongly required, since this works as a security measure against setting any invalid locale from user input.

config.i18n.load_path

Sets the path Rails uses to look for locale files. Defaults to config/locales/**/*.{yml,rb}.

config.i18n.raise_on_missing_translations

Determines whether an error should be raised for missing translations. If true, views and controllers raise I18n::MissingTranslationData. If :strict, models also raise the error. This defaults to false.

config.i18n.fallbacks

Sets fallback behavior for missing translations. Here are 3 usage examples for this option:

Configuring Active Model

config.active_model.i18n_customize_full_message

Controls whether the Error#full_message format can be overridden in an i18n locale file. Defaults to false.

When set to true, full_message will look for a format at the attribute and model level of the locale files. The default format is "%{attribute} %{message}", where attribute is the name of the attribute, and message is the validation-specific message. The following example overrides the format for all Person attributes, as well as the format for a specific Person attribute (age).

class Person
  include ActiveModel::Validations

  attr_accessor :name, :age

  validates :name, :age, presence: true
end
en:
  activemodel: # or activerecord:
    errors:
      models:
        person:
          # Override the format for all Person attributes:
          format: "Invalid %{attribute} (%{message})"
          attributes:
            age:
              # Override the format for the age attribute:
              format: "%{message}"
              blank: "Please fill in your %{attribute}"

irb> person = Person.new.tap(&:valid?)

irb> person.errors.full_messages
=> [
  "Invalid Name (can't be blank)",
  "Please fill in your Age"
]

irb> person.errors.messages
=> {
  :name => ["can't be blank"],
  :age  => ["Please fill in your Age"]
}

Configuring Active Record

config.active_record includes a variety of configuration options:

config.active_record.logger

Accepts a logger conforming to the interface of Log4r or the default Ruby Logger class, which is then passed on to any new database connections made. You can retrieve this logger by calling logger on either an Active Record model class or an Active Record model instance. Set to nil to disable logging.

config.active_record.primary_key_prefix_type

Lets you adjust the naming for primary key columns. By default, Rails assumes that primary key columns are named id (and this configuration option doesn't need to be set). There are two other choices:

config.active_record.table_name_prefix

Lets you set a global string to be prepended to table names. If you set this to northwest_, then the Customer class will look for northwest_customers as its table. The default is an empty string.

config.active_record.table_name_suffix

Lets you set a global string to be appended to table names. If you set this to _northwest, then the Customer class will look for customers_northwest as its table. The default is an empty string.

config.active_record.schema_migrations_table_name

Lets you set a string to be used as the name of the schema migrations table.

config.active_record.internal_metadata_table_name

Lets you set a string to be used as the name of the internal metadata table.

config.active_record.protected_environments

Lets you set an array of names of environments where destructive actions should be prohibited.

config.active_record.pluralize_table_names

Specifies whether Rails will look for singular or plural table names in the database. If set to true (the default), then the Customer class will use the customers table. If set to false, then the Customer class will use the customer table.

config.active_record.default_timezone

Determines whether to use Time.local (if set to :local) or Time.utc (if set to :utc) when pulling dates and times from the database. The default is :utc.

config.active_record.schema_format

Controls the format for dumping the database schema to a file. The options are :ruby (the default) for a database-independent version that depends on migrations, or :sql for a set of (potentially database-dependent) SQL statements.

config.active_record.error_on_ignored_order

Specifies if an error should be raised if the order of a query is ignored during a batch query. The options are true (raise error) or false (warn). Default is false.

config.active_record.timestamped_migrations

Controls whether migrations are numbered with serial integers or with timestamps. The default is true, to use timestamps, which are preferred if there are multiple developers working on the same application.

config.active_record.automatically_invert_plural_associations

Controls whether Active Record will automatically look for inverse relations with a pluralized name.

Example:

class Post < ApplicationRecord
  has_many :comments
end

class Comment < ApplicationRecord
  belongs_to :post
end

In the above case Active Record used to only look for a :comment (singular) association in Post, and won't find it.

With this option enabled, it will also look for a :comments association. In the vast majority of cases having the inverse association discovered is beneficial as it can prevent some useless queries, but it may cause backward compatibility issues with legacy code that doesn't expect it.

This behavior can be disabled on a per-model basis:

class Comment < ApplicationRecord
  self.automatically_invert_plural_associations = false

  belongs_to :post
end

And on a per-association basis:

class Comment < ApplicationRecord
  self.automatically_invert_plural_associations = true

  belongs_to :post, inverse_of: nil
end
Starting with version The default value is
(original) false

config.active_record.validate_migration_timestamps

Controls whether to validate migration timestamps. When set, an error will be raised if the timestamp prefix for a migration is more than a day ahead of the timestamp associated with the current time. This is done to prevent forward-dating of migration files, which can impact migration generation and other migration commands. config.active_record.timestamped_migrations must be set to true.

The default value depends on the config.load_defaults target version:

Starting with version The default value is
(original) false
7.2 true

config.active_record.db_warnings_action

Controls the action to be taken when an SQL query produces a warning. The following options are available:

config.active_record.db_warnings_ignore

Specifies an allowlist of warning codes and messages that will be ignored, regardless of the configured db_warnings_action. The default behavior is to report all warnings. Warnings to ignore can be specified as Strings or Regexps. For example:

config.active_record.db_warnings_action = :raise
# The following warnings will not be raised
config.active_record.db_warnings_ignore = [
  /Invalid utf8mb4 character string/,
  "An exact warning message",
  "1062", # MySQL Error 1062: Duplicate entry
]

config.active_record.migration_strategy

Controls the strategy class used to perform schema statement methods in a migration. The default class delegates to the connection adapter. Custom strategies should inherit from ::ActiveRecord::Migration::ExecutionStrategy, or may inherit from DefaultStrategy, which will preserve the default behaviour for methods that aren't implemented:

class CustomMigrationStrategy < ActiveRecord::Migration::DefaultStrategy
  def drop_table(*)
    raise "Dropping tables is not supported!"
  end
end

config.active_record.migration_strategy = CustomMigrationStrategy

config.active_record.lock_optimistically

Controls whether Active Record will use optimistic locking and is true by default.

config.active_record.cache_timestamp_format

Controls the format of the timestamp value in the cache key. Default is :usec.

config.active_record.record_timestamps

Is a boolean value which controls whether or not timestamping of create and update operations on a model occur. The default value is true.

config.active_record.partial_inserts

Is a boolean value and controls whether or not partial writes are used when creating new records (i.e. whether inserts only set attributes that are different from the default).

The default value depends on the config.load_defaults target version:

Starting with version The default value is
(original) true
7.0 false

config.active_record.partial_updates

Is a boolean value and controls whether or not partial writes are used when updating existing records (i.e. whether updates only set attributes that are dirty). Note that when using partial updates, you should also use optimistic locking config.active_record.lock_optimistically since concurrent updates may write attributes based on a possibly stale read state. The default value is true.

config.active_record.maintain_test_schema

Is a boolean value which controls whether Active Record should try to keep your test database schema up-to-date with db/schema.rb (or db/structure.sql) when you run your tests. The default is true.

config.active_record.dump_schema_after_migration

Is a flag which controls whether or not schema dump should happen (db/schema.rb or db/structure.sql) when you run migrations. This is set to false in config/environments/production.rb which is generated by Rails. The default value is true if this configuration is not set.

config.active_record.dump_schemas

Controls which database schemas will be dumped when calling db:schema:dump. The options are :schema_search_path (the default) which dumps any schemas listed in schema_search_path, :all which always dumps all schemas regardless of the schema_search_path, or a string of comma separated schemas.

config.active_record.before_committed_on_all_records

Enable before_committed! callbacks on all enrolled records in a transaction. The previous behavior was to only run the callbacks on the first copy of a record if there were multiple copies of the same record enrolled in the transaction.

Starting with version The default value is
(original) false
7.1 true

config.active_record.belongs_to_required_by_default

Is a boolean value and controls whether a record fails validation if belongs_to association is not present.

The default value depends on the config.load_defaults target version:

Starting with version The default value is
(original) nil
5.0 true

config.active_record.belongs_to_required_validates_foreign_key

Enable validating only parent-related columns for presence when the parent is mandatory. The previous behavior was to validate the presence of the parent record, which performed an extra query to get the parent every time the child record was updated, even when parent has not changed.

Starting with version The default value is
(original) true
7.1 false

config.active_record.marshalling_format_version

When set to 7.1, enables a more efficient serialization of Active Record instance with Marshal.dump.

This changes the serialization format, so models serialized this way cannot be read by older (< 7.1) versions of Rails. However, messages that use the old format can still be read, regardless of whether this optimization is enabled.

Starting with version The default value is
(original) 6.1
7.1 7.1

config.active_record.action_on_strict_loading_violation

Enables raising or logging an exception if strict_loading is set on an association. The default value is :raise in all environments. It can be changed to :log to send violations to the logger instead of raising.

config.active_record.strict_loading_by_default

Is a boolean value that either enables or disables strict_loading mode by default. Defaults to false.

config.active_record.strict_loading_mode

Sets the mode in which strict loading is reported. Defaults to :all. It can be changed to :n_plus_one_only to only report when loading associations that will lead to an N + 1 query.

config.active_record.index_nested_attribute_errors

Allows errors for nested has_many relationships to be displayed with an index as well as the error. Defaults to false.

config.active_record.use_schema_cache_dump

Enables users to get schema cache information from db/schema_cache.yml (generated by bin/rails db:schema:cache:dump), instead of having to send a query to the database to get this information. Defaults to true.

config.active_record.cache_versioning

Indicates whether to use a stable #cache_key method that is accompanied by a changing version in the #cache_version method.

The default value depends on the config.load_defaults target version:

Starting with version The default value is
(original) false
5.2 true

config.active_record.collection_cache_versioning

Enables the same cache key to be reused when the object being cached of type ::ActiveRecord::Relation changes by moving the volatile information (max updated at and count) of the relation's cache key into the cache version to support recycling cache key.

The default value depends on the config.load_defaults target version:

Starting with version The default value is
(original) false
6.0 true

config.active_record.has_many_inversing

Enables setting the inverse record when traversing belongs_to to has_many associations.

The default value depends on the config.load_defaults target version:

Starting with version The default value is
(original) false
6.1 true

config.active_record.automatic_scope_inversing

Enables automatically inferring the inverse_of for associations with a scope.

The default value depends on the config.load_defaults target version:

Starting with version The default value is
(original) false
7.0 true

config.active_record.destroy_association_async_job

Allows specifying the job that will be used to destroy the associated records in background. It defaults to ::ActiveRecord::DestroyAssociationAsyncJob.

config.active_record.destroy_association_async_batch_size

Allows specifying the maximum number of records that will be destroyed in a background job by the dependent: :destroy_async association option. All else equal, a lower batch size will enqueue more, shorter-running background jobs, while a higher batch size will enqueue fewer, longer-running background jobs. This option defaults to nil, which will cause all dependent records for a given association to be destroyed in the same background job.

config.active_record.queues.destroy

Allows specifying the Active Job queue to use for destroy jobs. When this option is nil, purge jobs are sent to the default Active Job queue (see config.active_job.default_queue_name). It defaults to nil.

config.active_record.enumerate_columns_in_select_statements

When true, will always include column names in SELECT statements, and avoid wildcard SELECT * FROM ... queries. This avoids prepared statement cache errors when adding columns to a PostgreSQL database for example. Defaults to false.

config.active_record.verify_foreign_keys_for_fixtures

Ensures all foreign key constraints are valid after fixtures are loaded in tests. Supported by PostgreSQL and SQLite only.

The default value depends on the config.load_defaults target version:

Starting with version The default value is
(original) false
7.0 true

config.active_record.raise_on_assign_to_attr_readonly

Enable raising on assignment to attr_readonly attributes. The previous behavior would allow assignment but silently not persist changes to the database.

Starting with version The default value is
(original) false
7.1 true

config.active_record.run_commit_callbacks_on_first_saved_instances_in_transaction

When multiple Active Record instances change the same record within a transaction, Rails runs after_commit or after_rollback callbacks for only one of them. This option specifies how Rails chooses which instance receives the callbacks.

When true, transactional callbacks are run on the first instance to save, even though its instance state may be stale.

When false, transactional callbacks are run on the instances with the freshest instance state. Those instances are chosen as follows:

The default value depends on the config.load_defaults target version:

Starting with version The default value is
(original) true
7.1 false

config.active_record.default_column_serializer

The serializer implementation to use if none is explicitly specified for a given column.

Historically serialize and store while allowing to use alternative serializer implementations, would use YAML by default, but it's not a very efficient format and can be the source of security vulnerabilities if not carefully employed.

As such it is recommended to prefer stricter, more limited formats for database serialization.

Unfortunately there isn't really any suitable defaults available in Ruby's standard library. JSON could work as a format, but the json gems will cast unsupported types to strings which may lead to bugs.

The default value depends on the config.load_defaults target version:

Starting with version The default value is
(original) YAML
7.1 nil

config.active_record.run_after_transaction_callbacks_in_order_defined

When true, after_commit callbacks are executed in the order they are defined in a model. When false, they are executed in reverse order.

All other callbacks are always executed in the order they are defined in a model (unless you use prepend: true).

The default value depends on the config.load_defaults target version:

Starting with version The default value is
(original) false
7.1 true

config.active_record.query_log_tags_enabled

Specifies whether or not to enable adapter-level query comments. Defaults to false.

NOTE: When this is set to true database prepared statements will be automatically disabled.

config.active_record.query_log_tags

Define an Array specifying the key/value tags to be inserted in an SQL comment. Defaults to [ :application, :controller, :action, :job ]. The available tags are: :application, :controller, :namespaced_controller, :action, :job, and :source_location.

config.active_record.query_log_tags_format

A Symbol specifying the formatter to use for tags. Valid values are :sqlcommenter and :legacy.

The default value depends on the config.load_defaults target version:

Starting with version The default value is
(original) :legacy
7.1 :sqlcommenter

config.active_record.cache_query_log_tags

Specifies whether or not to enable caching of query log tags. For applications that have a large number of queries, caching query log tags can provide a performance benefit when the context does not change during the lifetime of the request or job execution. Defaults to false.

config.active_record.schema_cache_ignored_tables

Define the list of table that should be ignored when generating the schema cache. It accepts an Array of strings, representing the table names, or regular expressions.

config.active_record.verbose_query_logs

Specifies if source locations of methods that call database queries should be logged below relevant queries. By default, the flag is true in development and false in all other environments.

config.active_record.sqlite3_adapter_strict_strings_by_default

Specifies whether the SQLite3Adapter should be used in a strict strings mode. The use of a strict strings mode disables double-quoted string literals.

SQLite has some quirks around double-quoted string literals. It first tries to consider double-quoted strings as identifier names, but if they don't exist it then considers them as string literals. Because of this, typos can silently go unnoticed. For example, it is possible to create an index for a non existing column. See SQLite documentation for more details.

The default value depends on the config.load_defaults target version:

Starting with version The default value is
(original) false
7.1 true

config.active_record.postgresql_adapter_decode_dates

Specifies whether the PostgresqlAdapter should decode date columns.

ActiveRecord::Base.connection
     .select_value("select '2024-01-01'::date").class #=> Date

The default value depends on the config.load_defaults target version:

Starting with version The default value is
(original) false
7.2 true

config.active_record.async_query_executor

Specifies how asynchronous queries are pooled.

It defaults to nil, which means load_async is disabled and instead directly executes queries in the foreground. For queries to actually be performed asynchronously, it must be set to either :global_thread_pool or :multi_thread_pool.

:global_thread_pool will use a single pool for all databases the application connects to. This is the preferred configuration for applications with only a single database, or applications which only ever query one database shard at a time.

:multi_thread_pool will use one pool per database, and each pool size can be configured individually in database.yml through the max_threads and min_thread properties. This can be useful to applications regularly querying multiple databases at a time, and that need to more precisely define the max concurrency.

config.active_record.global_executor_concurrency

Used in conjunction with config.active_record.async_query_executor = :global_thread_pool, defines how many asynchronous queries can be executed concurrently.

Defaults to 4.

This number must be considered in accordance with the database connection pool size configured in database.yml. The connection pool should be large enough to accommodate both the foreground threads (ie. web server or job worker threads) and background threads.

For each process, Rails will create one global query executor that uses this many threads to process async queries. Thus, the pool size should be at least thread_count + global_executor_concurrency + 1. For example, if your web server has a maximum of 3 threads, and global_executor_concurrency is set to 4, then your pool size should be at least 8.

config.active_record.yaml_column_permitted_classes

Defaults to [Symbol]. Allows applications to include additional permitted classes to safe_load() on the ::ActiveRecord::Coders::YAMLColumn.

config.active_record.use_yaml_unsafe_load

Defaults to false. Allows applications to opt into using unsafe_load on the ::ActiveRecord::Coders::YAMLColumn.

config.active_record.raise_int_wider_than_64bit

Defaults to true. Determines whether to raise an exception or not when the PostgreSQL adapter is provided an integer that is wider than signed 64bit representation.

config.active_record.generate_secure_token_on

Controls when to generate a value for has_secure_token declarations. By default, generate the value when the model is initialized:

class User < ApplicationRecord
  has_secure_token
end

record = User.new
record.token # => "fwZcXX6SkJBJRogzMdciS7wf"

With config.active_record.generate_secure_token_on = :create, generate the value when the model is created:

# config/application.rb

config.active_record.generate_secure_token_on = :create

# app/models/user.rb
class User < ApplicationRecord
  has_secure_token on: :create
end

record = User.new
record.token # => nil
record.save!
record.token # => "fwZcXX6SkJBJRogzMdciS7wf"
Starting with version The default value is
(original) :create
7.1 :initialize

config.active_record.permanent_connection_checkout

Controls whether ActiveRecord::Base.connection raises an error, emits a deprecation warning, or neither.

ActiveRecord::Base.connection checkouts a database connection from the pool and keeps it leased until the end of the request or job. This behavior can be undesirable in environments that use many more threads or fibers than there is available connections.

This configuration can be used to track down and eliminate code that calls ActiveRecord::Base.connection and migrate it to use ActiveRecord::Base.with_connection instead.

The value can be set to :disallowed, :deprecated, or true to respectively raise an error, emit a deprecation warning, or neither.

Starting with version The default value is
(original) true

config.active_record.database_cli

Controls which CLI tool will be used for accessing the database when running rails dbconsole. By default the standard tool for the database will be used (e.g. psql for PostgreSQL and mysql for MySQL). The option takes a hash which specifies the tool per-database system, and an array can be used where fallback options are required:

# config/application.rb

config.active_record.database_cli = { postgresql: "pgcli", mysql: %w[ mycli mysql ] }

ActiveRecord::ConnectionAdapters::Mysql2Adapter.emulate_booleans and ActiveRecord::ConnectionAdapters::TrilogyAdapter.emulate_booleans

Controls whether the Active Record MySQL adapter will consider all tinyint(1) columns as booleans. Defaults to true.

ActiveRecord::ConnectionAdapters::PostgreSQLAdapter.create_unlogged_tables

Controls whether database tables created by PostgreSQL should be "unlogged", which can speed up performance but adds a risk of data loss if the database crashes. It is highly recommended that you do not enable this in a production environment. Defaults to false in all environments.

To enable this for tests:

# config/environments/test.rb

ActiveSupport.on_load(:active_record_postgresqladapter) do
  self.create_unlogged_tables = true
end

ActiveRecord::ConnectionAdapters::PostgreSQLAdapter.datetime_type

Controls what native type the Active Record PostgreSQL adapter should use when you call datetime in a migration or schema. It takes a symbol which must correspond to one of the configured NATIVE_DATABASE_TYPES. The default is :timestamp, meaning t.datetime in a migration will create a "timestamp without time zone" column.

To use "timestamp with time zone":

# config/application.rb

ActiveSupport.on_load(:active_record_postgresqladapter) do
  self.datetime_type = :timestamptz
end

You should run bin/rails db:migrate to rebuild your schema.rb if you change this.

ActiveRecord::SchemaDumper.ignore_tables

Accepts an array of tables that should not be included in any generated schema file.

ActiveRecord::SchemaDumper.fk_ignore_pattern

Allows setting a different regular expression that will be used to decide whether a foreign key's name should be dumped to db/schema.rb or not. By default, foreign key names starting with fk_rails_ are not exported to the database schema dump. Defaults to /^fk_rails_[0-9a-f]{10}$/.

config.active_record.encryption.add_to_filter_parameters

Enables automatic filtering of encrypted attributes on inspect.

The default value is true.

config.active_record.encryption.hash_digest_class

Sets the digest algorithm used by Active Record Encryption.

The default value depends on the config.load_defaults target version:

Starting with version The default value is
(original) OpenSSL::Digest::SHA1
7.1 OpenSSL::Digest::SHA256

config.active_record.encryption.support_sha1_for_non_deterministic_encryption

Enables support for decrypting existing data encrypted using a SHA-1 digest class. When false, it will only support the digest configured in config.active_record.encryption.hash_digest_class.

The default value depends on the config.load_defaults target version:

Starting with version The default value is
(original) true
7.1 false

config.active_record.encryption.compressor

Sets the compressor used by Active Record Encryption. The default value is Zlib.

You can use your own compressor by setting this to a class that responds to deflate and inflate.

config.active_record.protocol_adapters

When using a URL to configure the database connection, this option provides a mapping from the protocol to the underlying database adapter. For example, this means the environment can specify DATABASE_URL=mysql://localhost/database and Rails will map mysql to the mysql2 adapter, but the application can also override these mappings:

config.active_record.protocol_adapters.mysql = "trilogy"

If no mapping is found, the protocol is used as the adapter name.

Configuring Action Controller

config.action_controller includes a number of configuration settings:

config.action_controller.asset_host

Sets the host for the assets. Useful when CDNs are used for hosting assets rather than the application server itself. You should only use this if you have a different configuration for Action Mailer, otherwise use config.asset_host.

config.action_controller.perform_caching

Configures whether the application should perform the caching features provided by the Action Controller component or not. Set to false in the development environment, true in production. If it's not specified, the default will be true.

config.action_controller.default_static_extension

Configures the extension used for cached pages. Defaults to .html.

config.action_controller.include_all_helpers

Configures whether all view helpers are available everywhere or are scoped to the corresponding controller. If set to false, UsersHelper methods are only available for views rendered as part of UsersController. If true, UsersHelper methods are available everywhere. The default configuration behavior (when this option is not explicitly set to true or false) is that all view helpers are available to each controller.

config.action_controller.logger

Accepts a logger conforming to the interface of Log4r or the default Ruby Logger class, which is then used to log information from Action Controller. Set to nil to disable logging.

config.action_controller.request_forgery_protection_token

Sets the token parameter name for RequestForgery. Calling protect_from_forgery sets it to :authenticity_token by default.

config.action_controller.allow_forgery_protection

Enables or disables CSRF protection. By default this is false in the test environment and true in all other environments.

config.action_controller.forgery_protection_origin_check

Configures whether the HTTP Origin header should be checked against the site's origin as an additional CSRF defense.

The default value depends on the config.load_defaults target version:

Starting with version The default value is
(original) false
5.0 true

config.action_controller.per_form_csrf_tokens

Configures whether CSRF tokens are only valid for the method/action they were generated for.

The default value depends on the config.load_defaults target version:

Starting with version The default value is
(original) false
5.0 true

config.action_controller.default_protect_from_forgery

Determines whether forgery protection is added on ::ActionController::Base.

The default value depends on the config.load_defaults target version:

Starting with version The default value is
(original) false
5.2 true

config.action_controller.relative_url_root

Can be used to tell Rails that you are deploying to a subdirectory. The default is config.relative_url_root.

config.action_controller.permit_all_parameters

Sets all the parameters for mass assignment to be permitted by default. The default value is false.

config.action_controller.action_on_unpermitted_parameters

Controls behavior when parameters that are not explicitly permitted are found. The default value is :log in test and development environments, false otherwise. The values can be:

config.action_controller.always_permitted_parameters

Sets a list of permitted parameters that are permitted by default. The default values are ['controller', 'action'].

config.action_controller.enable_fragment_cache_logging

Determines whether to log fragment cache reads and writes in verbose format as follows:

Read fragment views/v1/2914079/v1/2914079/recordings/70182313-20160225015037000000/d0bdf2974e1ef6d31685c3b392ad0b74 (0.6ms)
Rendered messages/_message.html.erb in 1.2 ms [cache hit]
Write fragment views/v1/2914079/v1/2914079/recordings/70182313-20160225015037000000/3b4e249ac9d168c617e32e84b99218b5 (1.1ms)
Rendered recordings/threads/_thread.html.erb in 1.5 ms [cache miss]

By default it is set to false which results in following output:

Rendered messages/_message.html.erb in 1.2 ms [cache hit]
Rendered recordings/threads/_thread.html.erb in 1.5 ms [cache miss]

config.action_controller.raise_on_missing_callback_actions

Raises an ::AbstractController::ActionNotFound when the action specified in callback's :only or :except options is missing in the controller.

Starting with version The default value is
(original) false
7.1 true (development and test), false (other envs)

config.action_controller.raise_on_open_redirects

Protect an application from unintentionally redirecting to an external host (also known as an "open redirect") by making external redirects opt-in.

When this configuration is set to true, an ::ActionController::Redirecting::UnsafeRedirectError will be raised when a URL with an external host is passed to redirect_to. If an open redirect should be allowed, then allow_other_host: true can be added to the call to redirect_to.

The default value depends on the config.load_defaults target version:

Starting with version The default value is
(original) false
7.0 true

config.action_controller.log_query_tags_around_actions

Determines whether controller context for query tags will be automatically updated via an around_filter. The default value is true.

config.action_controller.wrap_parameters_by_default

Before Rails 7.0, new applications were generated with an initializer named wrap_parameters.rb that enabled parameter wrapping in ::ActionController::Base for JSON requests.

Setting this configuration value to true has the same behavior as the initializer, allowing applications to remove the initializer if they do not wish to customize parameter wrapping behavior.

Regardless of this value, applications can continue to customize the parameter wrapping behavior as before in an initializer or per controller.

See ParamsWrapper for more information on parameter wrapping.

The default value depends on the config.load_defaults target version:

Starting with version The default value is
(original) false
7.0 true

ActionController::Base.wrap_parameters

Configures the ParamsWrapper. This can be called at the top level, or on individual controllers.

Configuring Action Dispatch

config.action_dispatch.cookies_serializer

Specifies which serializer to use for cookies. Accepts the same values as config.active_support.message_serializer, plus :hybrid which is an alias for :json_allow_marshal.

The default value depends on the config.load_defaults target version:

Starting with version The default value is
(original) :marshal
7.0 :json

config.action_dispatch.debug_exception_log_level

Configures the log level used by the ::ActionDispatch::DebugExceptions middleware when logging uncaught exceptions during requests.

The default value depends on the config.load_defaults target version:

Starting with version The default value is
(original) :fatal
7.1 :error

config.action_dispatch.default_headers

Is a hash with HTTP headers that are set by default in each response.

The default value depends on the config.load_defaults target version:

Starting with version The default value is
(original) 
{<br>  "X-Frame-Options" => "SAMEORIGIN",<br>  "X-XSS-Protection" => "1; mode=block",<br>  "X-Content-Type-Options" => "nosniff",<br>  "X-Download-Options" => "noopen",<br>  "X-Permitted-Cross-Domain-Policies" => "none",<br>  "Referrer-Policy" => "strict-origin-when-cross-origin"<br>}
7.0 
{<br>  "X-Frame-Options" => "SAMEORIGIN",<br>  "X-XSS-Protection" => "0",<br>  "X-Content-Type-Options" => "nosniff",<br>  "X-Download-Options" => "noopen",<br>  "X-Permitted-Cross-Domain-Policies" => "none",<br>  "Referrer-Policy" => "strict-origin-when-cross-origin"<br>}
7.1 
{<br>  "X-Frame-Options" => "SAMEORIGIN",<br>  "X-XSS-Protection" => "0",<br>  "X-Content-Type-Options" => "nosniff",<br>  "X-Permitted-Cross-Domain-Policies" => "none",<br>  "Referrer-Policy" => "strict-origin-when-cross-origin"<br>}

config.action_dispatch.default_charset

Specifies the default character set for all renders. Defaults to nil.

config.action_dispatch.tld_length

Sets the TLD (top-level domain) length for the application. Defaults to 1.

config.action_dispatch.ignore_accept_header

Is used to determine whether to ignore accept headers from a request. Defaults to false.

config.action_dispatch.x_sendfile_header

Specifies server specific X-Sendfile header. This is useful for accelerated file sending from server. For example it can be set to 'X-Sendfile' for Apache.

config.action_dispatch.http_auth_salt

Sets the HTTP Auth salt value. Defaults to 'http authentication'.

config.action_dispatch.signed_cookie_salt

Sets the signed cookies salt value. Defaults to 'signed cookie'.

config.action_dispatch.encrypted_cookie_salt

Sets the encrypted cookies salt value. Defaults to 'encrypted cookie'.

config.action_dispatch.encrypted_signed_cookie_salt

Sets the signed encrypted cookies salt value. Defaults to 'signed encrypted cookie'.

config.action_dispatch.authenticated_encrypted_cookie_salt

Sets the authenticated encrypted cookie salt. Defaults to 'authenticated encrypted cookie'.

config.action_dispatch.encrypted_cookie_cipher

Sets the cipher to be used for encrypted cookies. This defaults to "aes-256-gcm".

config.action_dispatch.signed_cookie_digest

Sets the digest to be used for signed cookies. This defaults to "SHA1".

config.action_dispatch.cookies_rotations

Allows rotating secrets, ciphers, and digests for encrypted and signed cookies.

config.action_dispatch.use_authenticated_cookie_encryption

Controls whether signed and encrypted cookies use the AES-256-GCM cipher or the older AES-256-CBC cipher.

The default value depends on the config.load_defaults target version:

Starting with version The default value is
(original) false
5.2 true

config.action_dispatch.use_cookies_with_metadata

Enables writing cookies with the purpose metadata embedded.

The default value depends on the config.load_defaults target version:

Starting with version The default value is
(original) false
6.0 true

config.action_dispatch.perform_deep_munge

Configures whether deep_munge method should be performed on the parameters. See Security Guide for more information. It defaults to true.

config.action_dispatch.rescue_responses

Configures what exceptions are assigned to an HTTP status. It accepts a hash and you can specify pairs of exception/status.

# It's good to use #[]= or #merge! to respect the default values
config.action_dispatch.rescue_responses["MyAuthenticationError"] = :unauthorized

Use ActionDispatch::ExceptionWrapper.rescue_responses to observe the configuration. By default, it is defined as:

{
  "ActionController::RoutingError" => :not_found,
  "AbstractController::ActionNotFound" => :not_found,
  "ActionController::MethodNotAllowed" => :method_not_allowed,
  "ActionController::UnknownHttpMethod" => :method_not_allowed,
  "ActionController::NotImplemented" => :not_implemented,
  "ActionController::UnknownFormat" => :not_acceptable,
  "ActionDispatch::Http::MimeNegotiation::InvalidType" => :not_acceptable,
  "ActionController::MissingExactTemplate" => :not_acceptable,
  "ActionController::InvalidAuthenticityToken" => :unprocessable_entity,
  "ActionController::InvalidCrossOriginRequest" => :unprocessable_entity,
  "ActionDispatch::Http::Parameters::ParseError" => :bad_request,
  "ActionController::BadRequest" => :bad_request,
  "ActionController::ParameterMissing" => :bad_request,
  "Rack::QueryParser::ParameterTypeError" => :bad_request,
  "Rack::QueryParser::InvalidParameterError" => :bad_request,
  "ActiveRecord::RecordNotFound" => :not_found,
  "ActiveRecord::StaleObjectError" => :conflict,
  "ActiveRecord::RecordInvalid" => :unprocessable_entity,
  "ActiveRecord::RecordNotSaved" => :unprocessable_entity
}

Any exceptions that are not configured will be mapped to 500 Internal Server Error.

config.action_dispatch.cookies_same_site_protection

Configures the default value of the SameSite attribute when setting cookies. When set to nil, the SameSite attribute is not added. To allow the value of the SameSite attribute to be configured dynamically based on the request, a proc may be specified. For example:

config.action_dispatch.cookies_same_site_protection = ->(request) do
  :strict unless request.user_agent == "TestAgent"
end

The default value depends on the config.load_defaults target version:

Starting with version The default value is
(original) nil
6.1 :lax

config.action_dispatch.ssl_default_redirect_status

Configures the default HTTP status code used when redirecting non-GET/HEAD requests from HTTP to HTTPS in the ::ActionDispatch::SSL middleware.

The default value depends on the config.load_defaults target version:

Starting with version The default value is
(original) 307
6.1 308

config.action_dispatch.log_rescued_responses

Enables logging those unhandled exceptions configured in rescue_responses. It defaults to true.

config.action_dispatch.show_exceptions

The config.action_dispatch.show_exceptions configuration controls how Action Pack (specifically the ::ActionDispatch::ShowExceptions middleware) handles exceptions raised while responding to requests.

Setting the value to :all configures Action Pack to rescue from exceptions and render corresponding error pages. For example, Action Pack would rescue from an ::ActiveRecord::RecordNotFound exception and render the contents of public/404.html with a 404 Not found status code.

Setting the value to :rescuable configures Action Pack rescue from exceptions defined in config.action_dispatch.rescue_responses, and raise all others. For example, Action Pack would rescue from ::ActiveRecord::RecordNotFound, but would raise a NoMethodError.

Setting the value to :none configures Action Pack raise all exceptions.

Starting with version The default value is
(original) true
7.1 :all

config.action_dispatch.strict_freshness

Configures whether the ActionDispatch::ETag middleware should prefer the ETag header over the Last-Modified header when both are present in the response.

If set to true, when both headers are present only the ETag is considered as specified by RFC 7232 section 6.

If set to false, when both headers are present, both headers are checked and both need to match for the response to be considered fresh.

Starting with version The default value is
(original) false
8.0 true

config.action_dispatch.always_write_cookie

Cookies will be written at the end of a request if they marked as insecure, if the request is made over SSL, or if the request is made to an onion service.

If set to true, cookies will be written even if this criteria is not met.

This defaults to true in development, and false in all other environments.

ActionDispatch::Callbacks.before

Takes a block of code to run before the request.

ActionDispatch::Callbacks.after

Takes a block of code to run after the request.

Configuring Action View

config.action_view includes a small number of configuration settings:

config.action_view.cache_template_loading

Controls whether or not templates should be reloaded on each request. Defaults to !config.enable_reloading.

config.action_view.field_error_proc

Provides an HTML generator for displaying errors that come from Active Model. The block is evaluated within the context of an Action View template. The default is

Proc.new { |html_tag, instance| content_tag :div, html_tag, class: "field_with_errors" }

config.action_view.default_form_builder

Tells Rails which form builder to use by default. The default is ::ActionView::Helpers::FormBuilder. If you want your form builder class to be loaded after initialization (so it's reloaded on each request in development), you can pass it as a String.

config.action_view.logger

Accepts a logger conforming to the interface of Log4r or the default Ruby Logger class, which is then used to log information from Action View. Set to nil to disable logging.

config.action_view.erb_trim_mode

Controls if certain ERB syntax should trim. It defaults to '-', which turns on trimming of tail spaces and newline when using <%= -%> or <%= =%>. Setting this to anything else will turn off trimming support.

config.action_view.frozen_string_literal

Compiles the ERB template with the # frozen_string_literal: true magic comment, making all string literals frozen and saving allocations. Set to true to enable it for all views.

config.action_view.embed_authenticity_token_in_remote_forms

Allows you to set the default behavior for authenticity_token in forms with remote: true. By default it's set to false, which means that remote forms will not include authenticity_token, which is helpful when you're fragment-caching the form. Remote forms get the authenticity from the meta tag, so embedding is unnecessary unless you support browsers without JavaScript. In such case you can either pass authenticity_token: true as a form option or set this config setting to true.

config.action_view.prefix_partial_path_with_controller_namespace

Determines whether or not partials are looked up from a subdirectory in templates rendered from namespaced controllers. For example, consider a controller named Admin::ArticlesController which renders this template:

<%= render @article %>

The default setting is true, which uses the partial at /admin/articles/_article.erb. Setting the value to false would render /articles/_article.erb, which is the same behavior as rendering from a non-namespaced controller such as ArticlesController.

config.action_view.automatically_disable_submit_tag

Determines whether submit_tag should automatically disable on click, this defaults to true.

config.action_view.debug_missing_translation

Determines whether to wrap the missing translations key in a <span> tag or not. This defaults to true.

config.action_view.form_with_generates_remote_forms

Determines whether form_with generates remote forms or not.

The default value depends on the config.load_defaults target version:

Starting with version The default value is
5.1 true
6.1 false

config.action_view.form_with_generates_ids

Determines whether form_with generates ids on inputs.

The default value depends on the config.load_defaults target version:

Starting with version The default value is
(original) false
5.2 true

config.action_view.default_enforce_utf8

Determines whether forms are generated with a hidden tag that forces older versions of Internet Explorer to submit forms encoded in UTF-8.

The default value depends on the config.load_defaults target version:

Starting with version The default value is
(original) true
6.0 false

config.action_view.image_loading

Specifies a default value for the loading attribute of <img> tags rendered by the image_tag helper. For example, when set to "lazy", <img> tags rendered by image_tag will include loading="lazy", which instructs the browser to wait until an image is near the viewport to load it. (This value can still be overridden per image by passing e.g. loading: "eager" to image_tag.) Defaults to nil.

config.action_view.image_decoding

Specifies a default value for the decoding attribute of <img> tags rendered by the image_tag helper. Defaults to nil.

config.action_view.annotate_rendered_view_with_filenames

Determines whether to annotate rendered view with template file names. This defaults to false.

config.action_view.preload_links_header

Determines whether javascript_include_tag and stylesheet_link_tag will generate a link header that preload assets.

The default value depends on the config.load_defaults target version:

Starting with version The default value is
(original) nil
6.1 true

config.action_view.button_to_generates_button_tag

When false, button_to will render a <button> or an <input> inside a <form> depending on how content is passed (<form> omitted for brevity):

<%= button_to "Content", "/" %>
# => <input type="submit" value="Content">

<%= button_to "/" do %>
  Content
<% end %>
# => <button type="submit">Content</button>

Setting this value to true makes button_to generate a <button> tag inside the <form> in both cases.

The default value depends on the config.load_defaults target version:

Starting with version The default value is
(original) false
7.0 true

config.action_view.apply_stylesheet_media_default

Determines whether stylesheet_link_tag will render screen as the default value for the media attribute when it's not provided.

The default value depends on the config.load_defaults target version:

Starting with version The default value is
(original) true
7.0 false

config.action_view.prepend_content_exfiltration_prevention

Determines whether or not the form_tag and button_to helpers will produce HTML tags prepended with browser-safe (but technically invalid) HTML that guarantees their contents cannot be captured by any preceding unclosed tags. The default value is false.

config.action_view.sanitizer_vendor

Configures the set of HTML sanitizers used by Action View by setting ActionView::Helpers::SanitizeHelper.sanitizer_vendor. The default value depends on the config.load_defaults target version:

Starting with version The default value is Which parses markup as
(original) Rails::HTML4::Sanitizer HTML4
7.1 Rails::HTML5::Sanitizer (see NOTE) HTML5

NOTE: Rails::HTML5::Sanitizer is not supported on JRuby, so on JRuby platforms Rails will fall back to Rails::HTML4::Sanitizer.

Configuring Action Mailbox

config.action_mailbox provides the following configuration options:

config.action_mailbox.logger

Contains the logger used by Action Mailbox. It accepts a logger conforming to the interface of Log4r or the default Ruby Logger class. The default is Rails.logger.

config.action_mailbox.logger = ActiveSupport::Logger.new(STDOUT)

config.action_mailbox.incinerate_after

Accepts an ::ActiveSupport::Duration indicating how long after processing ActionMailbox::InboundEmail records should be destroyed. It defaults to 30.days.

# Incinerate inbound emails 14 days after processing.
config.action_mailbox.incinerate_after = 14.days

config.action_mailbox.queues.incineration

Accepts a symbol indicating the Active Job queue to use for incineration jobs. When this option is nil, incineration jobs are sent to the default Active Job queue (see config.active_job.default_queue_name).

The default value depends on the config.load_defaults target version:

Starting with version The default value is
(original) :action_mailbox_incineration
6.1 nil

config.action_mailbox.queues.routing

Accepts a symbol indicating the Active Job queue to use for routing jobs. When this option is nil, routing jobs are sent to the default Active Job queue (see config.active_job.default_queue_name).

The default value depends on the config.load_defaults target version:

Starting with version The default value is
(original) :action_mailbox_routing
6.1 nil

config.action_mailbox.storage_service

Accepts a symbol indicating the Active Storage service to use for uploading emails. When this option is nil, emails are uploaded to the default Active Storage service (see config.active_storage.service).

Configuring Action Mailer

There are a number of settings available on config.action_mailer:

config.action_mailer.asset_host

Sets the host for the assets. Useful when CDNs are used for hosting assets rather than the application server itself. You should only use this if you have a different configuration for Action Controller, otherwise use config.asset_host.

config.action_mailer.logger

Accepts a logger conforming to the interface of Log4r or the default Ruby Logger class, which is then used to log information from Action Mailer. Set to nil to disable logging.

config.action_mailer.smtp_settings

Allows detailed configuration for the :smtp delivery method. It accepts a hash of options, which can include any of these options:

Additionally, it is possible to pass any configuration option Mail::SMTP respects.

config.action_mailer.smtp_timeout

Prior to version 2.8.0, the mail gem did not configure any default timeouts for its SMTP requests. This configuration enables applications to configure default values for both :open_timeout and :read_timeout in the mail gem so that requests do not end up stuck indefinitely.

The default value depends on the config.load_defaults target version:

Starting with version The default value is
(original) nil
7.0 5

config.action_mailer.sendmail_settings

Allows detailed configuration for the :sendmail delivery method. It accepts a hash of options, which can include any of these options:

config.action_mailer.file_settings

Configures the :file delivery method. It accepts a hash of options, which can include:

config.action_mailer.raise_delivery_errors

Specifies whether to raise an error if email delivery cannot be completed. It defaults to true.

config.action_mailer.delivery_method

Defines the delivery method and defaults to :smtp. See the configuration section in the Action Mailer guide for more info.

config.action_mailer.perform_deliveries

Specifies whether mail will actually be delivered and is true by default. It can be convenient to set it to false for testing.

config.action_mailer.default_options

Configures Action Mailer defaults. Use to set options like from or reply_to for every mailer. These default to:

{
  mime_version:  "1.0",
  charset:       "UTF-8",
  content_type: "text/plain",
  parts_order:  ["text/plain", "text/enriched", "text/html"]
}

Assign a hash to set additional options:

config.action_mailer.default_options = {
  from: "noreply@example.com"
}

config.action_mailer.observers

Registers observers which will be notified when mail is delivered.

config.action_mailer.observers = ["MailObserver"]

config.action_mailer.interceptors

Registers interceptors which will be called before mail is sent.

config.action_mailer.interceptors = ["MailInterceptor"]

config.action_mailer.preview_interceptors

Registers interceptors which will be called before mail is previewed.

config.action_mailer.preview_interceptors = ["MyPreviewMailInterceptor"]

config.action_mailer.preview_paths

Specifies the locations of mailer previews. Appending paths to this configuration option will cause those paths to be used in the search for mailer previews.

config.action_mailer.preview_paths << "#{Rails.root}/lib/mailer_previews"

config.action_mailer.show_previews

Enable or disable mailer previews. By default this is true in development.

config.action_mailer.show_previews = false

config.action_mailer.perform_caching

Specifies whether the mailer templates should perform fragment caching or not. If it's not specified, the default will be true.

config.action_mailer.deliver_later_queue_name

Specifies the Active Job queue to use for the default delivery job (see config.action_mailer.delivery_job). When this option is set to nil, delivery jobs are sent to the default Active Job queue (see config.active_job.default_queue_name).

Mailer classes can override this to use a different queue. Note that this only applies when using the default delivery job. If your mailer is using a custom job, its queue will be used.

Ensure that your Active Job adapter is also configured to process the specified queue, otherwise delivery jobs may be silently ignored.

The default value depends on the config.load_defaults target version:

Starting with version The default value is
(original) :mailers
6.1 nil

config.action_mailer.delivery_job

Specifies delivery job for mail.

The default value depends on the config.load_defaults target version:

Starting with version The default value is
(original) ::ActionMailer::MailDeliveryJob
6.0 "ActionMailer::MailDeliveryJob"

Configuring Active Support

There are a few configuration options available in Active Support:

config.active_support.bare

Enables or disables the loading of active_support/all when booting Rails. Defaults to nil, which means active_support/all is loaded.

config.active_support.test_order

Sets the order in which the test cases are executed. Possible values are :random and :sorted. Defaults to :random.

config.active_support.escape_html_entities_in_json

Enables or disables the escaping of HTML entities in JSON serialization. Defaults to true.

config.active_support.use_standard_json_time_format

Enables or disables serializing dates to ISO 8601 format. Defaults to true.

config.active_support.time_precision

Sets the precision of JSON encoded time values. Defaults to 3.

config.active_support.hash_digest_class

Allows configuring the digest class to use to generate non-sensitive digests, such as the ETag header.

The default value depends on the config.load_defaults target version:

Starting with version The default value is
(original) OpenSSL::Digest::MD5
5.2 OpenSSL::Digest::SHA1
7.0 OpenSSL::Digest::SHA256

config.active_support.key_generator_hash_digest_class

Allows configuring the digest class to use to derive secrets from the configured secret base, such as for encrypted cookies.

The default value depends on the config.load_defaults target version:

Starting with version The default value is
(original) OpenSSL::Digest::SHA1
7.0 OpenSSL::Digest::SHA256

config.active_support.use_authenticated_message_encryption

Specifies whether to use AES-256-GCM authenticated encryption as the default cipher for encrypting messages instead of AES-256-CBC.

The default value depends on the config.load_defaults target version:

Starting with version The default value is
(original) false
5.2 true

config.active_support.message_serializer

Specifies the default serializer used by ::ActiveSupport::MessageEncryptor and ::ActiveSupport::MessageVerifier instances. To make migrating between serializers easier, the provided serializers include a fallback mechanism to support multiple deserialization formats:

Serializer Serialize and deserialize Fallback deserialize
:marshal Marshal ::ActiveSupport::JSON, ::ActiveSupport::MessagePack
:json ::ActiveSupport::JSON ::ActiveSupport::MessagePack
:json_allow_marshal ::ActiveSupport::JSON ::ActiveSupport::MessagePack, Marshal
:message_pack ::ActiveSupport::MessagePack ::ActiveSupport::JSON
:message_pack_allow_marshal ::ActiveSupport::MessagePack ::ActiveSupport::JSON, Marshal

WARNING: Marshal is a potential vector for deserialization attacks in cases where a message signing secret has been leaked. If possible, choose a serializer that does not support Marshal.

INFO: The :message_pack and :message_pack_allow_marshal serializers support roundtripping some Ruby types that are not supported by JSON, such as Symbol. They can also provide improved performance and smaller payload sizes. However, they require the msgpack gem.

Each of the above serializers will emit a message_serializer_fallback.active_support event notification when they fall back to an alternate deserialization format, allowing you to track how often such fallbacks occur.

Alternatively, you can specify any serializer object that responds to dump and load methods. For example:

config.active_support.message_serializer = YAML

The default value depends on the config.load_defaults target version:

Starting with version The default value is
(original) :marshal
7.1 :json_allow_marshal

config.active_support.use_message_serializer_for_metadata

When true, enables a performance optimization that serializes message data and metadata together. This changes the message format, so messages serialized this way cannot be read by older (< 7.1) versions of Rails. However, messages that use the old format can still be read, regardless of whether this optimization is enabled.

The default value depends on the config.load_defaults target version:

Starting with version The default value is
(original) false
7.1 true

config.active_support.cache_format_version

Specifies which serialization format to use for the cache. Possible values are 7.0, and 7.1.

7.0 serializes cache entries more efficiently.

7.1 further improves efficiency, and allows expired and version-mismatched cache entries to be detected without deserializing their values. It also includes an optimization for bare string values such as view fragments.

All formats are backward and forward compatible, meaning cache entries written in one format can be read when using another format. This behavior makes it easy to migrate between formats without invalidating the entire cache.

The default value depends on the config.load_defaults target version:

Starting with version The default value is
7.0 7.0
7.1 7.1

config.active_support.deprecation

Configures the behavior of deprecation warnings. See Deprecation::Behavior for a description of the available options.

In the default generated config/environments files, this is set to :log for development and :stderr for test, and it is omitted for production in favor of config.active_support.report_deprecations.

config.active_support.disallowed_deprecation

Configures the behavior of disallowed deprecation warnings. See Deprecation::Behavior for a description of the available options.

In the default generated config/environments files, this is set to :raise for both development and test, and it is omitted for production in favor of config.active_support.report_deprecations.

config.active_support.disallowed_deprecation_warnings

Configures deprecation warnings that the Application considers disallowed. This allows, for example, specific deprecations to be treated as hard failures.

config.active_support.report_deprecations

When false, disables all deprecation warnings, including disallowed deprecations, from the application’s deprecators. This includes all the deprecations from Rails and other gems that may add their deprecator to the collection of deprecators, but may not prevent all deprecation warnings emitted from ActiveSupport::Deprecation.

In the default generated config/environments files, this is set to false for production.

config.active_support.isolation_level

Configures the locality of most of Rails internal state. If you use a fiber based server or job processor (e.g. falcon), you should set it to :fiber. Otherwise it is best to use :thread locality. Defaults to :thread.

config.active_support.executor_around_test_case

Configure the test suite to call Rails.application.executor.wrap around test cases. This makes test cases behave closer to an actual request or job. Several features that are normally disabled in test, such as Active Record query cache and asynchronous queries will then be enabled.

The default value depends on the config.load_defaults target version:

Starting with version The default value is
(original) false
7.0 true

config.active_support.to_time_preserves_timezone

Specifies whether to_time methods preserve the UTC offset of their receivers or preserves the timezone. If set to :zone, to_time methods will use the timezone of their receivers. If set to :offset, to_time methods will use the UTC offset. If false, to_time methods will convert to the local system UTC offset instead.

The default value depends on the config.load_defaults target version:

Starting with version The default value is
(original) false
5.0 :offset
8.0 :zone

ActiveSupport::Logger.silencer

Is set to false to disable the ability to silence logging in a block. The default is true.

ActiveSupport::Cache::Store.logger

Specifies the logger to use within cache store operations.

ActiveSupport.utc_to_local_returns_utc_offset_times

Configures ActiveSupport::TimeZone#utc_to_local to return a time with a UTC offset instead of a UTC time incorporating that offset.

The default value depends on the config.load_defaults target version:

Starting with version The default value is
(original) false
6.1 true

config.active_support.raise_on_invalid_cache_expiration_time

Specifies if an ArgumentError should be raised if Rails.cache fetch or write are given an invalid expires_at or expires_in time.

Options are true, and false. If false, the exception will be reported as handled and logged instead.

The default value depends on the config.load_defaults target version:

Starting with version The default value is
(original) false
7.1 true

Configuring Active Job

config.active_job provides the following configuration options:

config.active_job.queue_adapter

Sets the adapter for the queuing backend. The default adapter is :async. For an up-to-date list of built-in adapters see the ActiveJob::QueueAdapters API documentation.

# Be sure to have the adapter's gem in your Gemfile
# and follow the adapter's specific installation
# and deployment instructions.
config.active_job.queue_adapter = :sidekiq

config.active_job.default_queue_name

Can be used to change the default queue name. By default this is "default".

config.active_job.default_queue_name = :medium_priority

config.active_job.queue_name_prefix

Allows you to set an optional, non-blank, queue name prefix for all jobs. By default it is blank and not used.

The following configuration would queue the given job on the production_high_priority queue when run in production:

config.active_job.queue_name_prefix = Rails.env
class GuestsCleanupJob < ActiveJob::Base
  queue_as :high_priority
  #....
end

config.active_job.queue_name_delimiter

Has a default value of '_'. If queue_name_prefix is set, then queue_name_delimiter joins the prefix and the non-prefixed queue name.

The following configuration would queue the provided job on the video_server.low_priority queue:

# prefix must be set for delimiter to be used
config.active_job.queue_name_prefix = "video_server"
config.active_job.queue_name_delimiter = "."
class EncoderJob < ActiveJob::Base
  queue_as :low_priority
  #....
end

config.active_job.logger

Accepts a logger conforming to the interface of Log4r or the default Ruby Logger class, which is then used to log information from Active Job. You can retrieve this logger by calling logger on either an Active Job class or an Active Job instance. Set to nil to disable logging.

config.active_job.custom_serializers

Allows to set custom argument serializers. Defaults to [].

config.active_job.log_arguments

Controls if the arguments of a job are logged. Defaults to true.

config.active_job.verbose_enqueue_logs

Specifies if source locations of methods that enqueue background jobs should be logged below relevant enqueue log lines. By default, the flag is true in development and false in all other environments.

config.active_job.retry_jitter

Controls the amount of "jitter" (random variation) applied to the delay time calculated when retrying failed jobs.

The default value depends on the config.load_defaults target version:

Starting with version The default value is
(original) 0.0
6.1 0.15

config.active_job.log_query_tags_around_perform

Determines whether job context for query tags will be automatically updated via an around_perform. The default value is true.

Configuring Action Cable

config.action_cable.url

Accepts a string for the URL for where you are hosting your Action Cable server. You would use this option if you are running Action Cable servers that are separated from your main application.

config.action_cable.mount_path

Accepts a string for where to mount Action Cable, as part of the main server process. Defaults to /cable. You can set this as nil to not mount Action Cable as part of your normal Rails server.

You can find more detailed configuration options in the Action Cable Overview.

config.action_cable.precompile_assets

Determines whether the Action Cable assets should be added to the asset pipeline precompilation. It has no effect if Sprockets is not used. The default value is true.

config.action_cable.allow_same_origin_as_host

Determines whether an origin matching the cable server itself will be permitted. The default value is true.

Set to false to disable automatic access for same-origin requests, and strictly allow only the configured origins.

config.action_cable.allowed_request_origins

Determines the request origins which will be accepted but the cable server. The default value is /https?:\/\/localhost:\d+/ in the development environment.

Configuring Active Storage

config.active_storage provides the following configuration options:

config.active_storage.variant_processor

Accepts a symbol :mini_magick or :vips, specifying whether variant transformations and blob analysis will be performed with MiniMagick or ruby-vips.

The default value depends on the config.load_defaults target version:

Starting with version The default value is
(original) :mini_magick
7.0 :vips

config.active_storage.analyzers

Accepts an array of classes indicating the analyzers available for Active Storage blobs. By default, this is defined as:

config.active_storage.analyzers = [ActiveStorage::Analyzer::ImageAnalyzer::Vips, ActiveStorage::Analyzer::ImageAnalyzer::ImageMagick, ActiveStorage::Analyzer::VideoAnalyzer, ActiveStorage::Analyzer::AudioAnalyzer]

The image analyzers can extract width and height of an image blob; the video analyzer can extract width, height, duration, angle, aspect ratio, and presence/absence of video/audio channels of a video blob; the audio analyzer can extract duration and bit rate of an audio blob.

config.active_storage.previewers

Accepts an array of classes indicating the image previewers available in Active Storage blobs. By default, this is defined as:

config.active_storage.previewers = [ActiveStorage::Previewer::PopplerPDFPreviewer, ActiveStorage::Previewer::MuPDFPreviewer, ActiveStorage::Previewer::VideoPreviewer]

PopplerPDFPreviewer and MuPDFPreviewer can generate a thumbnail from the first page of a PDF blob; VideoPreviewer from the relevant frame of a video blob.

config.active_storage.paths

Accepts a hash of options indicating the locations of previewer/analyzer commands. The default is {}, meaning the commands will be looked for in the default path. Can include any of these options:

config.active_storage.paths[:ffprobe] = "/usr/local/bin/ffprobe"

config.active_storage.variable_content_types

Accepts an array of strings indicating the content types that Active Storage can transform through the variant processor. By default, this is defined as:

config.active_storage.variable_content_types = %w(image/png image/gif image/jpeg image/tiff image/bmp image/vnd.adobe.photoshop image/vnd.microsoft.icon image/webp image/avif image/heic image/heif)

config.active_storage.web_image_content_types

Accepts an array of strings regarded as web image content types in which variants can be processed without being converted to the fallback PNG format. For example, if you want to use AVIF variants in your application you can add image/avif to this array.

The default value depends on the config.load_defaults target version:

Starting with version The default value is
(original) %w(image/png image/jpeg image/gif)
7.2 %w(image/png image/jpeg image/gif image/webp)

config.active_storage.content_types_to_serve_as_binary

Accepts an array of strings indicating the content types that Active Storage will always serve as an attachment, rather than inline. By default, this is defined as:

config.active_storage.content_types_to_serve_as_binary = %w(text/html image/svg+xml application/postscript application/x-shockwave-flash text/xml application/xml application/xhtml+xml application/mathml+xml text/cache-manifest)

config.active_storage.content_types_allowed_inline

Accepts an array of strings indicating the content types that Active Storage allows to serve as inline. By default, this is defined as:

config.active_storage.content_types_allowed_inline = %w(image/webp image/avif image/png image/gif image/jpeg image/tiff image/vnd.adobe.photoshop image/vnd.microsoft.icon application/pdf)

config.active_storage.queues.analysis

Accepts a symbol indicating the Active Job queue to use for analysis jobs. When this option is nil, analysis jobs are sent to the default Active Job queue (see config.active_job.default_queue_name).

The default value depends on the config.load_defaults target version:

Starting with version The default value is
6.0 :active_storage_analysis
6.1 nil

config.active_storage.queues.mirror

Accepts a symbol indicating the Active Job queue to use for direct upload mirroring jobs. When this option is nil, mirroring jobs are sent to the default Active Job queue (see config.active_job.default_queue_name). The default is nil.

config.active_storage.queues.preview_image

Accepts a symbol indicating the Active Job queue to use for preprocessing previews of images. When this option is nil, jobs are sent to the default Active Job queue (see config.active_job.default_queue_name). The default is nil.

config.active_storage.queues.purge

Accepts a symbol indicating the Active Job queue to use for purge jobs. When this option is nil, purge jobs are sent to the default Active Job queue (see config.active_job.default_queue_name).

The default value depends on the config.load_defaults target version:

Starting with version The default value is
6.0 :active_storage_purge
6.1 nil

config.active_storage.queues.transform

Accepts a symbol indicating the Active Job queue to use for preprocessing variants. When this option is nil, jobs are sent to the default Active Job queue (see config.active_job.default_queue_name). The default is nil.

config.active_storage.logger

Can be used to set the logger used by Active Storage. Accepts a logger conforming to the interface of Log4r or the default Ruby Logger class.

config.active_storage.logger = ActiveSupport::Logger.new(STDOUT)

config.active_storage.service_urls_expire_in

Determines the default expiry of URLs generated by:

The default is 5 minutes.

config.active_storage.urls_expire_in

Determines the default expiry of URLs in the Rails application generated by Active Storage. The default is nil.

config.active_storage.touch_attachment_records

Directs ActiveStorage::Attachments to touch its corresponding record when updated. The default is true.

config.active_storage.routes_prefix

Can be used to set the route prefix for the routes served by Active Storage. Accepts a string that will be prepended to the generated routes.

config.active_storage.routes_prefix = "/files"

The default is /rails/active_storage.

config.active_storage.track_variants

Determines whether variants are recorded in the database.

The default value depends on the config.load_defaults target version:

Starting with version The default value is
(original) false
6.1 true

config.active_storage.draw_routes

Can be used to toggle Active Storage route generation. The default is true.

config.active_storage.resolve_model_to_route

Can be used to globally change how Active Storage files are delivered.

Allowed values are:

The default is :rails_storage_redirect.

config.active_storage.video_preview_arguments

Can be used to alter the way ffmpeg generates video preview images.

The default value depends on the config.load_defaults target version:

Starting with version The default value is
(original) "-y -vframes 1 -f image2"
7.0 "-vf 'select=eq(n\\,0)+eq(key\\,1)+gt(scene\\,0.015)"1
+ ",loop=loop=-1:size=2,trim=start_frame=1'"2
+ " -frames:v 1 -f image2"

  1. Select the first video frame, plus keyframes, plus frames that meet the scene change threshold.
  2. Use the first video frame as a fallback when no other frames meet the criteria by looping the first (one or) two selected frames, then dropping the first looped frame.

config.active_storage.multiple_file_field_include_hidden

In Rails 7.1 and beyond, Active Storage has_many_attached relationships will default to replacing the current collection instead of appending to it. Thus to support submitting an empty collection, when multiple_file_field_include_hidden is true, the file_field helper will render an auxiliary hidden field, similar to the auxiliary field rendered by the checkbox helper.

The default value depends on the config.load_defaults target version:

Starting with version The default value is
(original) false
7.0 true

config.active_storage.precompile_assets

Determines whether the Active Storage assets should be added to the asset pipeline precompilation. It has no effect if Sprockets is not used. The default value is true.

Configuring Action Text

config.action_text.attachment_tag_name

Accepts a string for the HTML tag used to wrap attachments. Defaults to "action-text-attachment".

config.action_text.sanitizer_vendor

Configures the HTML sanitizer used by Action Text by setting ActionText::ContentHelper.sanitizer to an instance of the class returned from the vendor's .safe_list_sanitizer method. The default value depends on the config.load_defaults target version:

Starting with version The default value is Which parses markup as
(original) Rails::HTML4::Sanitizer HTML4
7.1 Rails::HTML5::Sanitizer (see NOTE) HTML5

NOTE: Rails::HTML5::Sanitizer is not supported on JRuby, so on JRuby platforms Rails will fall back to Rails::HTML4::Sanitizer.

Regexp.timeout

See Ruby's documentation for Regexp.timeout=.

Configuring a Database

Just about every Rails application will interact with a database. You can connect to the database by setting an environment variable ENV['DATABASE_URL'] or by using a configuration file called config/database.yml.

Using the config/database.yml file you can specify all the information needed to access your database:

development:
  adapter: postgresql
  database: blog_development
  pool: 5

This will connect to the database named blog_development using the postgresql adapter. This same information can be stored in a URL and provided via an environment variable like this:

ENV["DATABASE_URL"] # => "postgresql://localhost/blog_development?pool=5"

The config/database.yml file contains sections for three different environments in which Rails can run by default:

If you wish, you can manually specify a URL inside of your config/database.yml

development:
  url: postgresql://localhost/blog_development?pool=5

The config/database.yml file can contain ERB tags <%= %>. Anything in the tags will be evaluated as Ruby code. You can use this to pull out data from an environment variable or to perform calculations to generate the needed connection information.

When using a ENV['DATABASE_URL'] or a url key in your config/database.yml file, Rails allows mapping the protocol in the URL to a database adapter that can be configured from within the application. This allows the adapter to be configured without modifying the URL set in the deployment environment. See: config.active_record.protocol_adapters.

TIP: You don't have to update the database configurations manually. If you look at the options of the application generator, you will see that one of the options is named --database. This option allows you to choose an adapter from a list of the most used relational databases. You can even run the generator repeatedly: cd .. && rails new blog --database=mysql. When you confirm the overwriting of the config/database.yml file, your application will be configured for MySQL instead of SQLite. Detailed examples of the common database connections are below.

Connection Preference

Since there are two ways to configure your connection (using config/database.yml or using an environment variable) it is important to understand how they can interact.

If you have an empty config/database.yml file but your ENV['DATABASE_URL'] is present, then Rails will connect to the database via your environment variable:

$ cat config/database.yml

$ echo $DATABASE_URL
postgresql://localhost/my_database

If you have a config/database.yml but no ENV['DATABASE_URL'] then this file will be used to connect to your database:

$ cat config/database.yml
development:
  adapter: postgresql
  database: my_database
  host: localhost

$ echo $DATABASE_URL

If you have both config/database.yml and ENV['DATABASE_URL'] set then Rails will merge the configuration together. To better understand this we must see some examples.

When duplicate connection information is provided the environment variable will take precedence:

$ cat config/database.yml
development:
  adapter: sqlite3
  database: NOT_my_database
  host: localhost

$ echo $DATABASE_URL
postgresql://localhost/my_database

$ bin/rails runner 'puts ActiveRecord::Base.configurations.inspect'
#<ActiveRecord::DatabaseConfigurations:0x00007fc8eab02880 @configurations=[
  #<ActiveRecord::DatabaseConfigurations::UrlConfig:0x00007fc8eab020b0
    @env_name="development", @spec_name="primary",
    @config={"adapter"=>"postgresql", "database"=>"my_database", "host"=>"localhost"}
    @url="postgresql://localhost/my_database">
  ]

Here the adapter, host, and database match the information in ENV['DATABASE_URL'].

If non-duplicate information is provided you will get all unique values, environment variable still takes precedence in cases of any conflicts.

$ cat config/database.yml
development:
  adapter: sqlite3
  pool: 5

$ echo $DATABASE_URL
postgresql://localhost/my_database

$ bin/rails runner 'puts ActiveRecord::Base.configurations.inspect'
#<ActiveRecord::DatabaseConfigurations:0x00007fc8eab02880 @configurations=[
  #<ActiveRecord::DatabaseConfigurations::UrlConfig:0x00007fc8eab020b0
    @env_name="development", @spec_name="primary",
    @config={"adapter"=>"postgresql", "database"=>"my_database", "host"=>"localhost", "pool"=>5}
    @url="postgresql://localhost/my_database">
  ]

Since pool is not in the ENV['DATABASE_URL'] provided connection information its information is merged in. Since adapter is duplicate, the ENV['DATABASE_URL'] connection information wins.

The only way to explicitly not use the connection information in ENV['DATABASE_URL'] is to specify an explicit URL connection using the "url" sub key:

$ cat config/database.yml
development:
  url: sqlite3:NOT_my_database

$ echo $DATABASE_URL
postgresql://localhost/my_database

$ bin/rails runner 'puts ActiveRecord::Base.configurations.inspect'
#<ActiveRecord::DatabaseConfigurations:0x00007fc8eab02880 @configurations=[
  #<ActiveRecord::DatabaseConfigurations::UrlConfig:0x00007fc8eab020b0
    @env_name="development", @spec_name="primary",
    @config={"adapter"=>"sqlite3", "database"=>"NOT_my_database"}
    @url="sqlite3:NOT_my_database">
  ]

Here the connection information in ENV['DATABASE_URL'] is ignored, note the different adapter and database name.

Since it is possible to embed ERB in your config/database.yml it is best practice to explicitly show you are using the ENV['DATABASE_URL'] to connect to your database. This is especially useful in production since you should not commit secrets like your database password into your source control (such as Git).

$ cat config/database.yml
production:
  url: <%= ENV['DATABASE_URL'] %>

Now the behavior is clear, that we are only using the connection information in ENV['DATABASE_URL'].

Configuring an SQLite3 Database

Rails comes with built-in support for SQLite3, which is a lightweight serverless database application. While Rails better configures SQLite for production workloads, a busy production environment may overload SQLite. Rails defaults to using an SQLite database when creating a new project, but you can always change it later.

Here's the section of the default configuration file (config/database.yml) with connection information for the development environment:

development:
  adapter: sqlite3
  database: storage/development.sqlite3
  pool: 5
  timeout: 5000

NOTE: Rails uses an SQLite3 database for data storage by default because it is a zero configuration database that just works. Rails also supports MySQL (including MariaDB) and PostgreSQL "out of the box", and has plugins for many database systems. If you are using a database in a production environment Rails most likely has an adapter for it.

Configuring a MySQL or MariaDB Database

If you choose to use MySQL or MariaDB instead of the shipped SQLite3 database, your config/database.yml will look a little different. Here's the development section:

development:
  adapter: mysql2
  encoding: utf8mb4
  database: blog_development
  pool: 5
  username: root
  password:
  socket: /tmp/mysql.sock

If your development database has a root user with an empty password, this configuration should work for you. Otherwise, change the username and password in the development section as appropriate.

NOTE: If your MySQL version is 5.5 or 5.6 and want to use the utf8mb4 character set by default, please configure your MySQL server to support the longer key prefix by enabling innodb_large_prefix system variable.

Advisory Locks are enabled by default on MySQL and are used to make database migrations concurrent safe. You can disable advisory locks by setting advisory_locks to false:

production:
  adapter: mysql2
  advisory_locks: false

Configuring a PostgreSQL Database

If you choose to use PostgreSQL, your config/database.yml will be customized to use PostgreSQL databases:

development:
  adapter: postgresql
  encoding: unicode
  database: blog_development
  pool: 5

By default Active Record uses database features like prepared statements and advisory locks. You might need to disable those features if you're using an external connection pooler like PgBouncer:

production:
  adapter: postgresql
  prepared_statements: false
  advisory_locks: false

If enabled, Active Record will create up to 1000 prepared statements per database connection by default. To modify this behavior you can set statement_limit to a different value:

production:
  adapter: postgresql
  statement_limit: 200

The more prepared statements in use: the more memory your database will require. If your PostgreSQL database is hitting memory limits, try lowering statement_limit or disabling prepared statements.

Configuring an SQLite3 Database for JRuby Platform

If you choose to use SQLite3 and are using JRuby, your config/database.yml will look a little different. Here's the development section:

development:
  adapter: jdbcsqlite3
  database: storage/development.sqlite3

Configuring a MySQL or MariaDB Database for JRuby Platform

If you choose to use MySQL or MariaDB and are using JRuby, your config/database.yml will look a little different. Here's the development section:

development:
  adapter: jdbcmysql
  database: blog_development
  username: root
  password:

Configuring a PostgreSQL Database for JRuby Platform

If you choose to use PostgreSQL and are using JRuby, your config/database.yml will look a little different. Here's the development section:

development:
  adapter: jdbcpostgresql
  encoding: unicode
  database: blog_development
  username: blog
  password:

Change the username and password in the development section as appropriate.

Configuring Metadata Storage

By default Rails will store information about your Rails environment and schema in an internal table named ar_internal_metadata.

To turn this off per connection, set use_metadata_table in your database configuration. This is useful when working with a shared database and/or database user that cannot create tables.

development:
  adapter: postgresql
  use_metadata_table: false

Configuring Retry Behaviour

By default, Rails will automatically reconnect to the database server and retry certain queries if something goes wrong. Only safely retryable (idempotent) queries will be retried. The number of retries can be specified in your the database configuration via connection_retries, or disabled by setting the value to 0. The default number of retries is 1.

development:
  adapter: mysql2
  connection_retries: 3

The database config also allows a retry_deadline to be configured. If a retry_deadline is configured, an otherwise-retryable query will not be retried if the specified time has elapsed while the query was first tried. For example, a retry_deadline of 5 seconds means that if 5 seconds have passed since a query was first attempted, we won't retry the query, even if it is idempotent and there are connection_retries left.

This value defaults to nil, meaning that all retryable queries are retried regardless of time elapsed. The value for this config should be specified in seconds.

development:
  adapter: mysql2
  retry_deadline: 5 # Stop retrying queries after 5 seconds

Configuring Query Cache

By default, Rails automatically caches the result sets returned by queries. If Rails encounters the same query again for that request or job, it will use the cached result set as opposed to running the query against the database again.

The query cache is stored in memory, and to avoid using too much memory, it automatically evicts the least recently used queries when reaching a threshold. By default the threshold is 100, but can be configured in the database.yml.

development:
  adapter: mysql2
  query_cache: 200

To entirely disable query caching, it can be set to false

development:
  adapter: mysql2
  query_cache: false

Creating Rails Environments

By default Rails ships with three environments: "development", "test", and "production". While these are sufficient for most use cases, there are circumstances when you want more environments.

Imagine you have a server which mirrors the production environment but is only used for testing. Such a server is commonly called a "staging server". To define an environment called "staging" for this server, just create a file called config/environments/staging.rb. Since this is a production-like environment, you could copy the contents of config/environments/production.rb as a starting point and make the necessary changes from there. It's also possible to require and extend other environment configurations like this:

# config/environments/staging.rb
require_relative "production"

Rails.application.configure do
  # Staging overrides
end

That environment is no different than the default ones, start a server with bin/rails server -e staging, a console with bin/rails console -e staging, Rails.env.staging? works, etc.

Deploy to a Subdirectory (relative URL root)

By default Rails expects that your application is running at the root (e.g. /). This section explains how to run your application inside a directory.

Let's assume we want to deploy our application to "/app1". Rails needs to know this directory to generate the appropriate routes:

config.relative_url_root = "/app1"

alternatively you can set the RAILS_RELATIVE_URL_ROOT environment variable.

Rails will now prepend "/app1" when generating links.

Using Passenger

Passenger makes it easy to run your application in a subdirectory. You can find the relevant configuration in the Passenger manual.

Using a Reverse Proxy

Deploying your application using a reverse proxy has definite advantages over traditional deploys. They allow you to have more control over your server by layering the components required by your application.

Many modern web servers can be used as a proxy server to balance third-party elements such as caching servers or application servers.

One such application server you can use is Unicorn to run behind a reverse proxy.

In this case, you would need to configure the proxy server (NGINX, Apache, etc) to accept connections from your application server (Unicorn). By default Unicorn will listen for TCP connections on port 8080, but you can change the port or configure it to use sockets instead.

You can find more information in the Unicorn readme and understand the philosophy behind it.

Once you've configured the application server, you must proxy requests to it by configuring your web server appropriately. For example your NGINX config may include:

upstream application_server {
  server 0.0.0.0:8080;
}

server {
  listen 80;
  server_name localhost;

  root /root/path/to/your_app/public;

  try_files $uri/index.html $uri.html @app;

  location @app {
    proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
    proxy_set_header Host $http_host;
    proxy_redirect off;
    proxy_pass http://application_server;
  }

  # some other configuration
}

Be sure to read the NGINX documentation for the most up-to-date information.

Rails Environment Settings

Some parts of Rails can also be configured externally by supplying environment variables. The following environment variables are recognized by various parts of Rails:

Using Initializer Files

After loading the framework and any gems in your application, Rails turns to loading initializers. An initializer is any Ruby file stored under config/initializers in your application. You can use initializers to hold configuration settings that should be made after all of the frameworks and gems are loaded, such as options to configure settings for these parts.

The files in config/initializers (and any subdirectories of config/initializers) are sorted and loaded one by one as part of the load_config_initializers initializer.

If an initializer has code that relies on code in another initializer, you can combine them into a single initializer instead. This makes the dependencies more explicit, and can help surface new concepts within your application. Rails also supports numbering of initializer file names, but this can lead to file name churn. Explicitly loading initializers with require is not recommended, since it will cause the initializer to get loaded twice.

NOTE: There is no guarantee that your initializers will run after all the gem initializers, so any initialization code that depends on a given gem having been initialized should go into a config.after_initialize block.

Load Hooks

Rails code can often be referenced on load of an application. Rails is responsible for the load order of these frameworks, so when you load frameworks, such as ::ActiveRecord::Base, prematurely you are violating an implicit contract your application has with Rails. Moreover, by loading code such as ::ActiveRecord::Base on boot of your application you are loading entire frameworks which may slow down your boot time and could cause conflicts with load order and boot of your application.

Load and configuration hooks are the API that allow you to hook into this initialization process without violating the load contract with Rails. This will also mitigate boot performance degradation and avoid conflicts.

Avoid Loading Rails Frameworks

Since Ruby is a dynamic language, some code will cause different Rails frameworks to load. Take this snippet for instance:

ActiveRecord::Base.include(MyActiveRecordHelper)

This snippet means that when this file is loaded, it will encounter ::ActiveRecord::Base. This encounter causes Ruby to look for the definition of that constant and will require it. This causes the entire Active Record framework to be loaded on boot.

ActiveSupport.on_load is a mechanism that can be used to defer the loading of code until it is actually needed. The snippet above can be changed to:

ActiveSupport.on_load(:active_record) do
  include MyActiveRecordHelper
end

This new snippet will only include MyActiveRecordHelper when ::ActiveRecord::Base is loaded.

When are Hooks called?

In the Rails framework these hooks are called when a specific library is loaded. For example, when ::ActionController::Base is loaded, the :action_controller_base hook is called. This means that all ActiveSupport.on_load calls with :action_controller_base hooks will be called in the context of ::ActionController::Base (that means self will be an ::ActionController::Base).

Modifying Code to Use Load Hooks

Modifying code is generally straightforward. If you have a line of code that refers to a Rails framework such as ::ActiveRecord::Base you can wrap that code in a load hook.

Modifying calls to include

ActiveRecord::Base.include(MyActiveRecordHelper)

becomes

ActiveSupport.on_load(:active_record) do
  # self refers to ActiveRecord::Base here,
  # so we can call .include
  include MyActiveRecordHelper
end

Modifying calls to prepend

ActionController::Base.prepend(MyActionControllerHelper)

becomes

ActiveSupport.on_load(:action_controller_base) do
  # self refers to ActionController::Base here,
  # so we can call .prepend
  prepend MyActionControllerHelper
end

Modifying calls to class methods

ActiveRecord::Base.include_root_in_json = true

becomes

ActiveSupport.on_load(:active_record) do
  # self refers to ActiveRecord::Base here
  self.include_root_in_json = true
end

Available Load Hooks

These are the load hooks you can use in your own code. To hook into the initialization process of one of the following classes use the available hook.

Class Hook
ActionCable action_cable
::ActionCable::Channel::Base action_cable_channel
::ActionCable::Connection::Base action_cable_connection
::ActionCable::Connection::TestCase action_cable_connection_test_case
::ActionController::API action_controller_api
::ActionController::API action_controller
::ActionController::Base action_controller_base
::ActionController::Base action_controller
::ActionController::TestCase action_controller_test_case
::ActionDispatch::IntegrationTest action_dispatch_integration_test
::ActionDispatch::Response action_dispatch_response
::ActionDispatch::Request action_dispatch_request
::ActionDispatch::SystemTestCase action_dispatch_system_test_case
::ActionMailbox::Base action_mailbox
ActionMailbox::InboundEmail action_mailbox_inbound_email
ActionMailbox::Record action_mailbox_record
::ActionMailbox::TestCase action_mailbox_test_case
::ActionMailer::Base action_mailer
::ActionMailer::TestCase action_mailer_test_case
::ActionText::Content action_text_content
ActionText::Record action_text_record
ActionText::RichText action_text_rich_text
ActionText::EncryptedRichText action_text_encrypted_rich_text
::ActionView::Base action_view
::ActionView::TestCase action_view_test_case
::ActiveJob::Base active_job
::ActiveJob::TestCase active_job_test_case
::ActiveModel::Model active_model
::ActiveModel::Translation active_model_translation
::ActiveRecord::Base active_record
::ActiveRecord::Encryption active_record_encryption
::ActiveRecord::TestFixtures active_record_fixtures
::ActiveRecord::ConnectionAdapters::PostgreSQLAdapter active_record_postgresqladapter
::ActiveRecord::ConnectionAdapters::Mysql2Adapter active_record_mysql2adapter
::ActiveRecord::ConnectionAdapters::TrilogyAdapter active_record_trilogyadapter
::ActiveRecord::ConnectionAdapters::SQLite3Adapter active_record_sqlite3adapter
ActiveStorage::Attachment active_storage_attachment
ActiveStorage::VariantRecord active_storage_variant_record
ActiveStorage::Blob active_storage_blob
ActiveStorage::Record active_storage_record
::ActiveSupport::TestCase active_support_test_case
i18n i18n

Initialization Events

Rails has 5 initialization events which can be hooked into (listed in the order that they are run):

To define an event for these hooks, use the block syntax within a ::Rails::Application, ::Rails::Railtie or ::Rails::Engine subclass:

module YourApp
  class Application < Rails::Application
    config.before_initialize do
      # initialization code goes here
    end
  end
end

Alternatively, you can also do it through the config method on the Rails.application object:

Rails.application.config.before_initialize do
  # initialization code goes here
end

WARNING: Some parts of your application, notably routing, are not yet set up at the point where the after_initialize block is called.

Rails::Railtie#initializer

Rails has several initializers that run on startup that are all defined by using the initializer method from ::Rails::Railtie. Here's an example of the set_helpers_path initializer from Action Controller:

initializer "action_controller.set_helpers_path" do |app|
  ActionController::Helpers.helpers_path = app.helpers_paths
end

The initializer method takes three arguments with the first being the name for the initializer and the second being an options hash (not shown here) and the third being a block. The :before key in the options hash can be specified to specify which initializer this new initializer must run before, and the :after key will specify which initializer to run this initializer after.

Initializers defined using the initializer method will be run in the order they are defined in, with the exception of ones that use the :before or :after methods.

WARNING: You may put your initializer before or after any other initializer in the chain, as long as it is logical. Say you have 4 initializers called "one" through "four" (defined in that order) and you define "four" to go before "two" but after "three", that just isn't logical and Rails will not be able to determine your initializer order.

The block argument of the initializer method is the instance of the application itself, and so we can access the configuration on it by using the config method as done in the example.

Because ::Rails::Application inherits from ::Rails::Railtie (indirectly), you can use the initializer method in config/application.rb to define initializers for the application.

Initializers

Below is a comprehensive list of all the initializers found in Rails in the order that they are defined (and therefore run in, unless otherwise stated).

Database Pooling

Active Record database connections are managed by ::ActiveRecord::ConnectionAdapters::ConnectionPool which ensures that a connection pool synchronizes the amount of thread access to a limited number of database connections. This limit defaults to 5 and can be configured in database.yml.

development:
  adapter: sqlite3
  database: storage/development.sqlite3
  pool: 5
  timeout: 5000

Since the connection pooling is handled inside of Active Record by default, all application servers (Thin, Puma, Unicorn, etc.) should behave the same. The database connection pool is initially empty. As demand for connections increases it will create them until it reaches the connection pool limit.

Any one request will check out a connection the first time it requires access to the database. At the end of the request it will check the connection back in. This means that the additional connection slot will be available again for the next request in the queue.

If you try to use more connections than are available, Active Record will block you and wait for a connection from the pool. If it cannot get a connection, a timeout error similar to that given below will be thrown.

ActiveRecord::ConnectionTimeoutError - could not obtain a database connection within 5.000 seconds (waited 5.000 seconds)

If you get the above error, you might want to increase the size of the connection pool by incrementing the pool option in database.yml

NOTE. If you are running in a multi-threaded environment, there could be a chance that several threads may be accessing multiple connections simultaneously. So depending on your current request load, you could very well have multiple threads contending for a limited number of connections.

Custom Configuration

You can configure your own code through the Rails configuration object with custom configuration under either the config.x namespace, or config directly. The key difference between these two is that you should be using config.x if you are defining nested configuration (ex: config.x.nested.hi), and just config for single level configuration (ex: config.hello).

config.x.payment_processing.schedule = :daily
config.x.payment_processing.retries  = 3
config.super_debugger = true

These configuration points are then available through the configuration object:

Rails.configuration.x.payment_processing.schedule # => :daily
Rails.configuration.x.payment_processing.retries  # => 3
Rails.configuration.x.payment_processing.not_set  # => nil
Rails.configuration.super_debugger                # => true

You can also use Rails::Application#config_for to load whole configuration files:

# config/payment.yml
production:
  environment: production
  merchant_id: production_merchant_id
  public_key:  production_public_key
  private_key: production_private_key

development:
  environment: sandbox
  merchant_id: development_merchant_id
  public_key:  development_public_key
  private_key: development_private_key

# config/application.rb
module MyApp
  class Application < Rails::Application
    config.payment = config_for(:payment)
  end
end
Rails.configuration.payment["merchant_id"] # => production_merchant_id or development_merchant_id

Rails::Application#config_for supports a shared configuration to group common configurations. The shared configuration will be merged into the environment configuration.

# config/example.yml
shared:
  foo:
    bar:
      baz: 1

development:
  foo:
    bar:
      qux: 2

# development environment
Rails.application.config_for(:example)[:foo][:bar] #=> { baz: 1, qux: 2 }

Search Engines Indexing

Sometimes, you may want to prevent some pages of your application to be visible on search sites like Google, Bing, Yahoo, or Duck Duck Go. The robots that index these sites will first analyze the http://your-site.com/robots.txt file to know which pages it is allowed to index.

Rails creates this file for you inside the /public folder. By default, it allows search engines to index all pages of your application. If you want to block indexing on all pages of your application, use this:

User-agent: *
Disallow: /

To block just specific pages, it's necessary to use a more complex syntax. Learn it on the official documentation.