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Active Support Core Extensions

Active Support is the Ruby on Rails component responsible for providing Ruby language extensions, utilities, and other transversal stuff.

It offers a richer bottom-line at the language level, targeted both at the development of Rails applications, and at the development of Ruby on Rails itself.

After reading this guide, you will know:

• What Core Extensions are.
• How to load all extensions.
• How to cherry-pick just the extensions you want.
• What extensions Active Support provides.

---

How to Load Core Extensions

Stand-Alone Active Support

In order to have a near-zero default footprint, Active Support does not load anything by default. It is broken in small pieces so that you can load just what you need, and also has some convenience entry points to load related extensions in one shot, even everything.

Thus, after a simple require like:

require 'active_support'

objects do not even respond to blank?. Let's see how to load its definition.

Cherry-picking a Definition

The most lightweight way to get blank? is to cherry-pick the file that defines it.

For every single method defined as a core extension this guide has a note that says where such a method is defined. In the case of blank? the note reads:

NOTE: Defined in active_support/core_ext/object/blank.rb.

That means that you can require it like this:

require 'active_support'
require 'active_support/core_ext/object/blank'

Active Support has been carefully revised so that cherry-picking a file loads only strictly needed dependencies, if any.

Loading Grouped Core Extensions

The next level is to simply load all extensions to Object. As a rule of thumb, extensions to SomeClass are available in one shot by loading active_support/core_ext/some_class.

Thus, to load all extensions to Object (including blank?):

require 'active_support'
require 'active_support/core_ext/object'

Loading All Core Extensions

You may prefer just to load all core extensions, there is a file for that:

require 'active_support'
require 'active_support/core_ext'

Loading All Active Support

And finally, if you want to have all Active Support available just issue:

require 'active_support/all'

That does not even put the entire Active Support in memory upfront indeed, some stuff is configured via autoload, so it is only loaded if used.

Active Support Within a Ruby on Rails Application

A Ruby on Rails application loads all Active Support unless config.active_support.bare is true. In that case, the application will only load what the framework itself cherry-picks for its own needs, and can still cherry-pick itself at any granularity level, as explained in the previous section.

Extensions to All Objects

blank? and present?

The following values are considered to be blank in a Rails application:

• nil and false,

• strings composed only of whitespace (see note below),

• empty arrays and hashes, and

• any other object that responds to empty? and is empty.

INFO: The predicate for strings uses the Unicode-aware character class [:space:], so for example U+2029 (paragraph separator) is considered to be whitespace.

WARNING: Note that numbers are not mentioned. In particular, 0 and 0.0 are not blank.

For example, this method from ::ActionController::HttpAuthentication::Token::ControllerMethods uses blank? for checking whether a token is present:

def authenticate(controller, &login_procedure)
  token, options = token_and_options(controller.request)
  unless token.blank?
    login_procedure.call(token, options)
  end
end

The method present? is equivalent to !blank?. This example is taken from ::ActionDispatch::Http::Cache::Response:

def set_conditional_cache_control!
  return if self["Cache-Control"].present?
  #...
end

NOTE: Defined in active_support/core_ext/object/blank.rb.

presence

The presence method returns its receiver if present?, and nil otherwise. It is useful for idioms like this:

host = config[:host].presence || 'localhost'

NOTE: Defined in active_support/core_ext/object/blank.rb.

duplicable?

In Ruby 2.4 most objects can be duplicated via dup or clone except methods and certain numbers. Though Ruby 2.2 and 2.3 can't duplicate nil, false, true, and symbols as well as instances Float, Fixnum, and Bignum instances.

"foo".dup           # => "foo"
"".dup              # => ""
1.method(:+).dup    # => TypeError: allocator undefined for Method
Complex(0).dup      # => TypeError: can't copy Complex

Active Support provides duplicable? to query an object about this:

"foo".duplicable?           # => true
"".duplicable?              # => true
Rational(1).duplicable?     # => false
Complex(1).duplicable?      # => false
1.method(:+).duplicable?    # => false

duplicable? matches Ruby's dup according to the Ruby version.

So in 2.4:

nil.dup                 # => nil
:my_symbol.dup          # => :my_symbol
1.dup                   # => 1

nil.duplicable?         # => true
:my_symbol.duplicable?  # => true
1.duplicable?           # => true

Whereas in 2.2 and 2.3:

nil.dup                 # => TypeError: can't dup NilClass
:my_symbol.dup          # => TypeError: can't dup Symbol
1.dup                   # => TypeError: can't dup Fixnum

nil.duplicable?         # => false
:my_symbol.duplicable?  # => false
1.duplicable?           # => false

WARNING: Any class can disallow duplication by removing dup and clone or raising exceptions from them. Thus only rescue can tell whether a given arbitrary object is duplicable. duplicable? depends on the hard-coded list above, but it is much faster than rescue. Use it only if you know the hard-coded list is enough in your use case.

NOTE: Defined in active_support/core_ext/object/duplicable.rb.

deep_dup

The deep_dup method returns a deep copy of a given object. Normally, when you dup an object that contains other objects, Ruby does not dup them, so it creates a shallow copy of the object. If you have an array with a string, for example, it will look like this:

array     = ['string']
duplicate = array.dup

duplicate.push 'another-string'

# the object was duplicated, so the element was added only to the duplicate
array     # => ['string']
duplicate # => ['string', 'another-string']

duplicate.first.gsub!('string', 'foo')

# first element was not duplicated, it will be changed in both arrays
array     # => ['foo']
duplicate # => ['foo', 'another-string']

As you can see, after duplicating the Array instance, we got another object, therefore we can modify it and the original object will stay unchanged. This is not true for array's elements, however. Since dup does not make deep copy, the string inside the array is still the same object.

If you need a deep copy of an object, you should use deep_dup. Here is an example:

array     = ['string']
duplicate = array.deep_dup

duplicate.first.gsub!('string', 'foo')

array     # => ['string']
duplicate # => ['foo']

If the object is not duplicable, deep_dup will just return it:

number = 1
duplicate = number.deep_dup
number.object_id == duplicate.object_id   # => true

NOTE: Defined in active_support/core_ext/object/deep_dup.rb.

try

When you want to call a method on an object only if it is not nil, the simplest way to achieve it is with conditional statements, adding unnecessary clutter. The alternative is to use try. try is like Object#send except that it returns nil if sent to nil.

Here is an example:

# without try
unless @number.nil?
  @number.next
end

# with try
@number.try(:next)

Another example is this code from ::ActiveRecord::ConnectionAdapters::AbstractAdapter where @logger could be nil. You can see that the code uses try and avoids an unnecessary check.

def log_info(sql, name, ms)
  if @logger.try(:debug?)
    name = '%s (%.1fms)' % [name || 'SQL', ms]
    @logger.debug(format_log_entry(name, sql.squeeze(' ')))
  end
end

try can also be called without arguments but a block, which will only be executed if the object is not nil:

@person.try { |p| "#{p.first_name} #{p.last_name}" }

Note that try will swallow no-method errors, returning nil instead. If you want to protect against typos, use try! instead:

@number.try(:nest)  # => nil
@number.try!(:nest) # NoMethodError: undefined method `nest' for 1:Integer

NOTE: Defined in active_support/core_ext/object/try.rb.

class_eval(*args, &block)

You can evaluate code in the context of any object's singleton class using class_eval:

class Proc
  def bind(object)
    block, time = self, Time.current
    object.class_eval do
      method_name = "__bind_#{time.to_i}_#{time.usec}"
      define_method(method_name, &block)
      method = instance_method(method_name)
      remove_method(method_name)
      method
    end.bind(object)
  end
end

NOTE: Defined in active_support/core_ext/kernel/singleton_class.rb.

acts_like?(duck)

The method acts_like? provides a way to check whether some class acts like some other class based on a simple convention: a class that provides the same interface as String defines

def acts_like_string?
end

which is only a marker, its body or return value are irrelevant. Then, client code can query for duck-type-safeness this way:

some_klass.acts_like?(:string)

Rails has classes that act like Date or Time and follow this contract.

NOTE: Defined in active_support/core_ext/object/acts_like.rb.

to_param

All objects in Rails respond to the method to_param, which is meant to return something that represents them as values in a query string, or as URL fragments.

By default to_param just calls to_s:

7.to_param # => "7"

The return value of to_param should not be escaped:

"Tom & Jerry".to_param # => "Tom & Jerry"

Several classes in Rails overwrite this method.

For example nil, true, and false return themselves. Array#to_param calls to_param on the elements and joins the result with "/":

[0, true, String].to_param # => "0/true/String"

Notably, the Rails routing system calls to_param on models to get a value for the :id placeholder. ActiveRecord::Base#to_param returns the id of a model, but you can redefine that method in your models. For example, given

class User
  def to_param
    "#{id}-#{name.parameterize}"
  end
end

we get:

user_path(@user) # => "/users/357-john-smith"

WARNING. Controllers need to be aware of any redefinition of to_param because when a request like that comes in "357-john-smith" is the value of params[:id].

NOTE: Defined in active_support/core_ext/object/to_param.rb.

to_query

Except for hashes, given an unescaped key this method constructs the part of a query string that would map such key to what to_param returns. For example, given

class User
  def to_param
    "#{id}-#{name.parameterize}"
  end
end

we get:

current_user.to_query('user') # => "user=357-john-smith"

This method escapes whatever is needed, both for the key and the value:

account.to_query('company[name]')
# => "company%5Bname%5D=Johnson%26Johnson"

so its output is ready to be used in a query string.

Arrays return the result of applying to_query to each element with key[] as key, and join the result with "&":

[3.4, -45.6].to_query('sample')
# => "sample%5B%5D=3.4&sample%5B%5D=-45.6"

Hashes also respond to to_query but with a different signature. If no argument is passed a call generates a sorted series of key/value assignments calling to_query(key) on its values. Then it joins the result with "&":

{c: 3, b: 2, a: 1}.to_query # => "a=1&b=2&c=3"

The method Hash#to_query accepts an optional namespace for the keys:

{id: 89, name: "John Smith"}.to_query('user')
# => "user%5Bid%5D=89&user%5Bname%5D=John+Smith"

NOTE: Defined in active_support/core_ext/object/to_query.rb.

with_options

The method with_options provides a way to factor out common options in a series of method calls.

Given a default options hash, with_options yields a proxy object to a block. Within the block, methods called on the proxy are forwarded to the receiver with their options merged. For example, you get rid of the duplication in:

class Account < ApplicationRecord
  has_many :customers, dependent: :destroy
  has_many :products,  dependent: :destroy
  has_many :invoices,  dependent: :destroy
  has_many :expenses,  dependent: :destroy
end

this way:

class Account < ApplicationRecord
  with_options dependent: :destroy do |assoc|
    assoc.has_many :customers
    assoc.has_many :products
    assoc.has_many :invoices
    assoc.has_many :expenses
  end
end

That idiom may convey grouping to the reader as well. For example, say you want to send a newsletter whose language depends on the user. Somewhere in the mailer you could group locale-dependent bits like this:

I18n.with_options locale: user.locale, scope: "newsletter" do |i18n|
  subject i18n.t :subject
  body    i18n.t :body, user_name: user.name
end

TIP: Since with_options forwards calls to its receiver they can be nested. Each nesting level will merge inherited defaults in addition to their own.

NOTE: Defined in active_support/core_ext/object/with_options.rb.

JSON support

Active Support provides a better implementation of to_json than the json gem ordinarily provides for Ruby objects. This is because some classes, like Hash, OrderedHash and ::Process::Status need special handling in order to provide a proper JSON representation.

NOTE: Defined in active_support/core_ext/object/json.rb.

Instance Variables

Active Support provides several methods to ease access to instance variables.

instance_values

The method instance_values returns a hash that maps instance variable names without "@" to their corresponding values. Keys are strings:

class C
  def initialize(x, y)
    @x, @y = x, y
  end
end

C.new(0, 1).instance_values # => {"x" => 0, "y" => 1}

NOTE: Defined in active_support/core_ext/object/instance_variables.rb.

instance_variable_names

The method instance_variable_names returns an array. Each name includes the "@" sign.

class C
  def initialize(x, y)
    @x, @y = x, y
  end
end

C.new(0, 1).instance_variable_names # => ["@x", "@y"]

NOTE: Defined in active_support/core_ext/object/instance_variables.rb.

Silencing Warnings and Exceptions

The methods silence_warnings and enable_warnings change the value of $VERBOSE accordingly for the duration of their block, and reset it afterwards:

silence_warnings { Object.const_set "RAILS_DEFAULT_LOGGER", logger }

Silencing exceptions is also possible with suppress. This method receives an arbitrary number of exception classes. If an exception is raised during the execution of the block and is kind_of? any of the arguments, suppress captures it and returns silently. Otherwise the exception is not captured:

# If the user is locked, the increment is lost, no big deal.
suppress(ActiveRecord::StaleObjectError) do
  current_user.increment! :visits
end

NOTE: Defined in active_support/core_ext/kernel/reporting.rb.

in?

The predicate in? tests if an object is included in another object. An ArgumentError exception will be raised if the argument passed does not respond to include?.

Examples of in?:

1.in?([1,2])        # => true
"lo".in?("hello")   # => true
25.in?(30..50)      # => false
1.in?(1)            # => ArgumentError

NOTE: Defined in active_support/core_ext/object/inclusion.rb.

Extensions to Module

Attributes

alias_attribute

Model attributes have a reader, a writer, and a predicate. You can alias a model attribute having the corresponding three methods defined for you in one shot. As in other aliasing methods, the new name is the first argument, and the old name is the second (one mnemonic is that they go in the same order as if you did an assignment):

class User < ApplicationRecord
  # You can refer to the email column as "login".
  # This can be meaningful for authentication code.
  alias_attribute :login, :email
end

NOTE: Defined in active_support/core_ext/module/aliasing.rb.

Internal Attributes

When you are defining an attribute in a class that is meant to be subclassed, name collisions are a risk. That's remarkably important for libraries.

Active Support defines the macros attr_internal_reader, attr_internal_writer, and attr_internal_accessor. They behave like their Ruby built-in attr_* counterparts, except they name the underlying instance variable in a way that makes collisions less likely.

The macro attr_internal is a synonym for attr_internal_accessor:

# library
class ThirdPartyLibrary::Crawler
  attr_internal :log_level
end

# client code
class MyCrawler < ThirdPartyLibrary::Crawler
  attr_accessor :log_level
end

In the previous example it could be the case that :log_level does not belong to the public interface of the library and it is only used for development. The client code, unaware of the potential conflict, subclasses and defines its own :log_level. Thanks to attr_internal there's no collision.

By default the internal instance variable is named with a leading underscore, @_log_level in the example above. That's configurable via Module.attr_internal_naming_format though, you can pass any sprintf-like format string with a leading @ and a %s somewhere, which is where the name will be placed. The default is "@_%s".

Rails uses internal attributes in a few spots, for examples for views:

module ActionView
  class Base
    attr_internal :captures
    attr_internal :request, :layout
    attr_internal :controller, :template
  end
end

NOTE: Defined in active_support/core_ext/module/attr_internal.rb.

Module Attributes

The macros mattr_reader, mattr_writer, and mattr_accessor are the same as the cattr_* macros defined for class. In fact, the cattr_* macros are just aliases for the mattr_* macros. Check Class Attributes.

For example, the dependencies mechanism uses them:

module ActiveSupport
  module Dependencies
    mattr_accessor :warnings_on_first_load
    mattr_accessor :history
    mattr_accessor :loaded
    mattr_accessor :mechanism
    mattr_accessor :load_paths
    mattr_accessor :load_once_paths
    mattr_accessor :autoloaded_constants
    mattr_accessor :explicitly_unloadable_constants
    mattr_accessor :constant_watch_stack
    mattr_accessor :constant_watch_stack_mutex
  end
end

NOTE: Defined in active_support/core_ext/module/attribute_accessors.rb.

Parents

parent

The parent method on a nested named module returns the module that contains its corresponding constant:

module X
  module Y
    module Z
    end
  end
end
M = X::Y::Z

X::Y::Z.parent # => X::Y
M.parent       # => X::Y

If the module is anonymous or belongs to the top-level, parent returns Object.

WARNING: Note that in that case parent_name returns nil.

NOTE: Defined in active_support/core_ext/module/introspection.rb.

parent_name

The parent_name method on a nested named module returns the fully qualified name of the module that contains its corresponding constant:

module X
  module Y
    module Z
    end
  end
end
M = X::Y::Z

X::Y::Z.parent_name # => "X::Y"
M.parent_name       # => "X::Y"

For top-level or anonymous modules parent_name returns nil.

WARNING: Note that in that case parent returns Object.

NOTE: Defined in active_support/core_ext/module/introspection.rb.

parents

The method parents calls parent on the receiver and upwards until Object is reached. The chain is returned in an array, from bottom to top:

module X
  module Y
    module Z
    end
  end
end
M = X::Y::Z

X::Y::Z.parents # => [X::Y, X, Object]
M.parents       # => [X::Y, X, Object]

NOTE: Defined in active_support/core_ext/module/introspection.rb.

Anonymous

A module may or may not have a name:

module M
end
M.name # => "M"

N = Module.new
N.name # => "N"

Module.new.name # => nil

You can check whether a module has a name with the predicate anonymous?:

module M
end
M.anonymous? # => false

Module.new.anonymous? # => true

Note that being unreachable does not imply being anonymous:

module M
end

m = Object.send(:remove_const, :M)

m.anonymous? # => false

though an anonymous module is unreachable by definition.

NOTE: Defined in active_support/core_ext/module/anonymous.rb.

Method Delegation

delegate

The macro delegate offers an easy way to forward methods.

Let's imagine that users in some application have login information in the User model but name and other data in a separate Profile model:

class User < ApplicationRecord
  has_one :profile
end

With that configuration you get a user's name via their profile, user.profile.name, but it could be handy to still be able to access such attribute directly:

class User < ApplicationRecord
  has_one :profile

  def name
    profile.name
  end
end

That is what delegate does for you:

class User < ApplicationRecord
  has_one :profile

  delegate :name, to: :profile
end

It is shorter, and the intention more obvious.

The method must be public in the target.

The delegate macro accepts several methods:

delegate :name, :age, :address, :twitter, to: :profile

When interpolated into a string, the :to option should become an expression that evaluates to the object the method is delegated to. Typically a string or symbol. Such an expression is evaluated in the context of the receiver:

# delegates to the Rails constant
delegate :logger, to: :Rails

# delegates to the receiver's class
delegate :table_name, to: :class

WARNING: If the :prefix option is true this is less generic, see below.

By default, if the delegation raises NoMethodError and the target is nil the exception is propagated. You can ask that nil is returned instead with the :allow_nil option:

delegate :name, to: :profile, allow_nil: true

With :allow_nil the call user.name returns nil if the user has no profile.

The option :prefix adds a prefix to the name of the generated method. This may be handy for example to get a better name:

delegate :street, to: :address, prefix: true

The previous example generates address_street rather than street.

WARNING: Since in this case the name of the generated method is composed of the target object and target method names, the :to option must be a method name.

A custom prefix may also be configured:

delegate :size, to: :attachment, prefix: :avatar

In the previous example the macro generates avatar_size rather than size.

NOTE: Defined in active_support/core_ext/module/delegation.rb

delegate_missing_to

Imagine you would like to delegate everything missing from the User object, to the Profile one. The delegate_missing_to macro lets you implement this in a breeze:

class User < ApplicationRecord
  has_one :profile

  delegate_missing_to :profile
end

The target can be anything callable within the object, e.g. instance variables, methods, constants, etc. Only the public methods of the target are delegated.

NOTE: Defined in active_support/core_ext/module/delegation.rb.

Redefining Methods

There are cases where you need to define a method with define_method, but don't know whether a method with that name already exists. If it does, a warning is issued if they are enabled. No big deal, but not clean either.

The method redefine_method prevents such a potential warning, removing the existing method before if needed.

You can also use silence_redefinition_of_method if you need to define the replacement method yourself (because you're using delegate, for example).

NOTE: Defined in active_support/core_ext/module/redefine_method.rb.

Extensions to Class

Class Attributes

class_attribute

The method class_attribute declares one or more inheritable class attributes that can be overridden at any level down the hierarchy.

class A
  class_attribute :x
end

class B < A; end

class C < B; end

A.x = :a
B.x # => :a
C.x # => :a

B.x = :b
A.x # => :a
C.x # => :b

C.x = :c
A.x # => :a
B.x # => :b

For example ::ActionMailer::Base defines:

class_attribute :default_params
self.default_params = {
  mime_version: "1.0",
  charset: "UTF-8",
  content_type: "text/plain",
  parts_order: [ "text/plain", "text/enriched", "text/html" ]
}.freeze

They can also be accessed and overridden at the instance level.

A.x = 1

a1 = A.new
a2 = A.new
a2.x = 2

a1.x # => 1, comes from A
a2.x # => 2, overridden in a2

The generation of the writer instance method can be prevented by setting the option :instance_writer to false.

module ActiveRecord
  class Base
    class_attribute :table_name_prefix, instance_writer: false, default: "my"
  end
end

A model may find that option useful as a way to prevent mass-assignment from setting the attribute.

The generation of the reader instance method can be prevented by setting the option :instance_reader to false.

class A
  class_attribute :x, instance_reader: false
end

A.new.x = 1
A.new.x # NoMethodError

For convenience class_attribute also defines an instance predicate which is the double negation of what the instance reader returns. In the examples above it would be called x?.

When :instance_reader is false, the instance predicate returns a NoMethodError just like the reader method.

If you do not want the instance predicate, pass instance_predicate: false and it will not be defined.

NOTE: Defined in active_support/core_ext/class/attribute.rb.

cattr_reader, cattr_writer, and cattr_accessor

The macros cattr_reader, cattr_writer, and cattr_accessor are analogous to their attr_* counterparts but for classes. They initialize a class variable to nil unless it already exists, and generate the corresponding class methods to access it:

class MysqlAdapter < AbstractAdapter
  # Generates class methods to access @@emulate_booleans.
  cattr_accessor :emulate_booleans, default: true
end

Instance methods are created as well for convenience, they are just proxies to the class attribute. So, instances can change the class attribute, but cannot override it as it happens with class_attribute (see above). For example given

module ActionView
  class Base
    cattr_accessor :field_error_proc, default: Proc.new { ... }
  end
end

we can access field_error_proc in views.

Also, you can pass a block to cattr_* to set up the attribute with a default value:

class MysqlAdapter < AbstractAdapter
  # Generates class methods to access @@emulate_booleans with default value of true.
  cattr_accessor :emulate_booleans, default: true
end

The generation of the reader instance method can be prevented by setting :instance_reader to false and the generation of the writer instance method can be prevented by setting :instance_writer to false. Generation of both methods can be prevented by setting :instance_accessor to false. In all cases, the value must be exactly false and not any false value.

module A
  class B
    # No first_name instance reader is generated.
    cattr_accessor :first_name, instance_reader: false
    # No last_name= instance writer is generated.
    cattr_accessor :last_name, instance_writer: false
    # No surname instance reader or surname= writer is generated.
    cattr_accessor :surname, instance_accessor: false
  end
end

A model may find it useful to set :instance_accessor to false as a way to prevent mass-assignment from setting the attribute.

NOTE: Defined in active_support/core_ext/module/attribute_accessors.rb.

Subclasses & Descendants

subclasses

The subclasses method returns the subclasses of the receiver:

class C; end
C.subclasses # => []

class B < C; end
C.subclasses # => [B]

class A < B; end
C.subclasses # => [B]

class D < C; end
C.subclasses # => [B, D]

The order in which these classes are returned is unspecified.

NOTE: Defined in active_support/core_ext/class/subclasses.rb.

descendants

The descendants method returns all classes that are < than its receiver:

class C; end
C.descendants # => []

class B < C; end
C.descendants # => [B]

class A < B; end
C.descendants # => [B, A]

class D < C; end
C.descendants # => [B, A, D]

The order in which these classes are returned is unspecified.

NOTE: Defined in active_support/core_ext/class/subclasses.rb.

Extensions to String

Output Safety

Motivation

Inserting data into HTML templates needs extra care. For example, you can't just interpolate @review.title verbatim into an HTML page. For one thing, if the review title is "Flanagan & Matz rules!" the output won't be well-formed because an ampersand has to be escaped as "&". What's more, depending on the application, that may be a big security hole because users can inject malicious HTML setting a hand-crafted review title. Check out the section about cross-site scripting in the Security guide for further information about the risks.

Safe Strings

Active Support has the concept of (html) safe strings. A safe string is one that is marked as being insertable into HTML as is. It is trusted, no matter whether it has been escaped or not.

Strings are considered to be unsafe by default:

"".html_safe? # => false

You can obtain a safe string from a given one with the html_safe method:

s = "".html_safe
s.html_safe? # => true

It is important to understand that html_safe performs no escaping whatsoever, it is just an assertion:

s = "<script>...</script>".html_safe
s.html_safe? # => true
s            # => "<script>...</script>"

It is your responsibility to ensure calling html_safe on a particular string is fine.

If you append onto a safe string, either in-place with concat/<<, or with +, the result is a safe string. Unsafe arguments are escaped:

"".html_safe + "<" # => "&lt;"

Safe arguments are directly appended:

"".html_safe + "<".html_safe # => "<"

These methods should not be used in ordinary views. Unsafe values are automatically escaped:

<%= @review.title %> <%# fine, escaped if needed %>

To insert something verbatim use the raw helper rather than calling html_safe:

<%= raw @cms.current_template %> <%# inserts @cms.current_template as is %>

or, equivalently, use <%==:

<%== @cms.current_template %> <%# inserts @cms.current_template as is %>

The raw helper calls html_safe for you:

def raw(stringish)
  stringish.to_s.html_safe
end

NOTE: Defined in active_support/core_ext/string/output_safety.rb.

Transformation

As a rule of thumb, except perhaps for concatenation as explained above, any method that may change a string gives you an unsafe string. These are downcase, gsub, strip, chomp, underscore, etc.

In the case of in-place transformations like gsub! the receiver itself becomes unsafe.

INFO: The safety bit is lost always, no matter whether the transformation actually changed something.

Conversion and Coercion

Calling to_s on a safe string returns a safe string, but coercion with to_str returns an unsafe string.

Copying

Calling dup or clone on safe strings yields safe strings.

remove

The method remove will remove all occurrences of the pattern:

"Hello World".remove(/Hello /) # => "World"

There's also the destructive version String#remove!.

NOTE: Defined in active_support/core_ext/string/filters.rb.

squish

The method squish strips leading and trailing whitespace, and substitutes runs of whitespace with a single space each:

" \n  foo\n\r \t bar \n".squish # => "foo bar"

There's also the destructive version String#squish!.

Note that it handles both ASCII and Unicode whitespace.

NOTE: Defined in active_support/core_ext/string/filters.rb.

truncate

The method truncate returns a copy of its receiver truncated after a given length:

"Oh dear! Oh dear! I shall be late!".truncate(20)
# => "Oh dear! Oh dear!..."

Ellipsis can be customized with the :omission option:

"Oh dear! Oh dear! I shall be late!".truncate(20, omission: '…')
# => "Oh dear! Oh …"

Note in particular that truncation takes into account the length of the omission string.

Pass a :separator to truncate the string at a natural break:

"Oh dear! Oh dear! I shall be late!".truncate(18)
# => "Oh dear! Oh dea..."
"Oh dear! Oh dear! I shall be late!".truncate(18, separator: ' ')
# => "Oh dear! Oh..."

The option :separator can be a regexp:

"Oh dear! Oh dear! I shall be late!".truncate(18, separator: /\s/)
# => "Oh dear! Oh..."

In above examples "dear" gets cut first, but then :separator prevents it.

NOTE: Defined in active_support/core_ext/string/filters.rb.

truncate_words

The method truncate_words returns a copy of its receiver truncated after a given number of words:

"Oh dear! Oh dear! I shall be late!".truncate_words(4)
# => "Oh dear! Oh dear!..."

Ellipsis can be customized with the :omission option:

"Oh dear! Oh dear! I shall be late!".truncate_words(4, omission: '…')
# => "Oh dear! Oh dear!…"

Pass a :separator to truncate the string at a natural break:

"Oh dear! Oh dear! I shall be late!".truncate_words(3, separator: '!')
# => "Oh dear! Oh dear! I shall be late..."

The option :separator can be a regexp:

"Oh dear! Oh dear! I shall be late!".truncate_words(4, separator: /\s/)
# => "Oh dear! Oh dear!..."

NOTE: Defined in active_support/core_ext/string/filters.rb.

inquiry

The inquiry method converts a string into a StringInquirer object making equality checks prettier.

"production".inquiry.production? # => true
"active".inquiry.inactive?       # => false

starts_with? and ends_with?

Active Support defines 3rd person aliases of String#start_with? and String#end_with?:

"foo".starts_with?("f") # => true
"foo".ends_with?("o")   # => true

NOTE: Defined in active_support/core_ext/string/starts_ends_with.rb.

strip_heredoc

The method strip_heredoc strips indentation in heredocs.

For example in

if options[:usage]
  puts <<-USAGE.strip_heredoc
    This command does such and such.

    Supported options are:
      -h         This message
      #...
  USAGE
end

the user would see the usage message aligned against the left margin.

Technically, it looks for the least indented line in the whole string, and removes that amount of leading whitespace.

NOTE: Defined in active_support/core_ext/string/strip.rb.

indent

Indents the lines in the receiver:

<<EOS.indent(2)
def some_method
  some_code
end
EOS
# =>
  def some_method
    some_code
  end

The second argument, indent_string, specifies which indent string to use. The default is nil, which tells the method to make an educated guess peeking at the first indented line, and fallback to a space if there is none.

"  foo".indent(2)        # => "    foo"
"foo\n\t\tbar".indent(2) # => "\t\tfoo\n\t\t\t\tbar"
"foo".indent(2, "\t")    # => "\t\tfoo"

While indent_string is typically one space or tab, it may be any string.

The third argument, indent_empty_lines, is a flag that says whether empty lines should be indented. Default is false.

"foo\n\nbar".indent(2)            # => "  foo\n\n  bar"
"foo\n\nbar".indent(2, nil, true) # => "  foo\n  \n  bar"

The indent! method performs indentation in-place.

NOTE: Defined in active_support/core_ext/string/indent.rb.

Access

at(position)

Returns the character of the string at position position:

"hello".at(0)  # => "h"
"hello".at(4)  # => "o"
"hello".at(-1) # => "o"
"hello".at(10) # => nil

NOTE: Defined in active_support/core_ext/string/access.rb.

from(position)

Returns the substring of the string starting at position position:

"hello".from(0)  # => "hello"
"hello".from(2)  # => "llo"
"hello".from(-2) # => "lo"
"hello".from(10) # => nil

NOTE: Defined in active_support/core_ext/string/access.rb.

to(position)

Returns the substring of the string up to position position:

"hello".to(0)  # => "h"
"hello".to(2)  # => "hel"
"hello".to(-2) # => "hell"
"hello".to(10) # => "hello"

NOTE: Defined in active_support/core_ext/string/access.rb.

first(limit = 1)

The call str.first(n) is equivalent to str.to(n-1) if n > 0, and returns an empty string for n == 0.

NOTE: Defined in active_support/core_ext/string/access.rb.

last(limit = 1)

The call str.last(n) is equivalent to str.from(-n) if n > 0, and returns an empty string for n == 0.

NOTE: Defined in active_support/core_ext/string/access.rb.

Inflections

pluralize

The method pluralize returns the plural of its receiver:

"table".pluralize     # => "tables"
"ruby".pluralize      # => "rubies"
"equipment".pluralize # => "equipment"

As the previous example shows, Active Support knows some irregular plurals and uncountable nouns. Built-in rules can be extended in config/initializers/inflections.rb. That file is generated by the rails command and has instructions in comments.

pluralize can also take an optional count parameter. If count == 1 the singular form will be returned. For any other value of count the plural form will be returned:

"dude".pluralize(0) # => "dudes"
"dude".pluralize(1) # => "dude"
"dude".pluralize(2) # => "dudes"

Active Record uses this method to compute the default table name that corresponds to a model:

# active_record/model_schema.rb
def undecorated_table_name(class_name = base_class.name)
  table_name = class_name.to_s.demodulize.underscore
  pluralize_table_names ? table_name.pluralize : table_name
end

NOTE: Defined in active_support/core_ext/string/inflections.rb.

singularize

The inverse of pluralize:

"tables".singularize    # => "table"
"rubies".singularize    # => "ruby"
"equipment".singularize # => "equipment"

Associations compute the name of the corresponding default associated class using this method:

# active_record/reflection.rb
def derive_class_name
  class_name = name.to_s.camelize
  class_name = class_name.singularize if collection?
  class_name
end

NOTE: Defined in active_support/core_ext/string/inflections.rb.

camelize

The method camelize returns its receiver in camel case:

"product".camelize    # => "Product"
"admin_user".camelize # => "AdminUser"

As a rule of thumb you can think of this method as the one that transforms paths into Ruby class or module names, where slashes separate namespaces:

"backoffice/session".camelize # => "Backoffice::Session"

For example, Action Pack uses this method to load the class that provides a certain session store:

# action_controller/metal/session_management.rb
def session_store=(store)
  @@session_store = store.is_a?(Symbol) ?
    ActionDispatch::Session.const_get(store.to_s.camelize) :
    store
end

camelize accepts an optional argument, it can be :upper (default), or :lower. With the latter the first letter becomes lowercase:

"visual_effect".camelize(:lower) # => "visualEffect"

That may be handy to compute method names in a language that follows that convention, for example JavaScript.

INFO: As a rule of thumb you can think of camelize as the inverse of underscore, though there are cases where that does not hold: "SSLError".underscore.camelize gives back "SslError". To support cases such as this, Active Support allows you to specify acronyms in config/initializers/inflections.rb:

ActiveSupport::Inflector.inflections do |inflect|
  inflect.acronym 'SSL'
end

"SSLError".underscore.camelize # => "SSLError"

camelize is aliased to camelcase.

NOTE: Defined in active_support/core_ext/string/inflections.rb.

underscore

The method underscore goes the other way around, from camel case to paths:

"Product".underscore   # => "product"
"AdminUser".underscore # => "admin_user"

Also converts "::" back to "/":

"Backoffice::Session".underscore # => "backoffice/session"

and understands strings that start with lowercase:

"visualEffect".underscore # => "visual_effect"

underscore accepts no argument though.

Rails class and module autoloading uses underscore to infer the relative path without extension of a file that would define a given missing constant:

# active_support/dependencies.rb
def load_missing_constant(from_mod, const_name)
  #...
  qualified_name = qualified_name_for from_mod, const_name
  path_suffix = qualified_name.underscore
  #...
end

INFO: As a rule of thumb you can think of underscore as the inverse of camelize, though there are cases where that does not hold. For example, "SSLError".underscore.camelize gives back "SslError".

NOTE: Defined in active_support/core_ext/string/inflections.rb.

titleize

The method titleize capitalizes the words in the receiver:

"alice in wonderland".titleize # => "Alice In Wonderland"
"fermat's enigma".titleize     # => "Fermat's Enigma"

titleize is aliased to titlecase.

NOTE: Defined in active_support/core_ext/string/inflections.rb.

dasherize

The method dasherize replaces the underscores in the receiver with dashes:

"name".dasherize         # => "name"
"contact_data".dasherize # => "contact-data"

The XML serializer of models uses this method to dasherize node names:

# active_model/serializers/xml.rb
def reformat_name(name)
  name = name.camelize if camelize?
  dasherize? ? name.dasherize : name
end

NOTE: Defined in active_support/core_ext/string/inflections.rb.

demodulize

Given a string with a qualified constant name, demodulize returns the very constant name, that is, the rightmost part of it:

"Product".demodulize                        # => "Product"
"Backoffice::UsersController".demodulize    # => "UsersController"
"Admin::Hotel::ReservationUtils".demodulize # => "ReservationUtils"
"::Inflections".demodulize                  # => "Inflections"
"".demodulize                               # => ""

Active Record for example uses this method to compute the name of a counter cache column:

# active_record/reflection.rb
def counter_cache_column
  if options[:counter_cache] == true
    "#{active_record.name.demodulize.underscore.pluralize}_count"
  elsif options[:counter_cache]
    options[:counter_cache]
  end
end

NOTE: Defined in active_support/core_ext/string/inflections.rb.

deconstantize

Given a string with a qualified constant reference expression, deconstantize removes the rightmost segment, generally leaving the name of the constant's container:

"Product".deconstantize                        # => ""
"Backoffice::UsersController".deconstantize    # => "Backoffice"
"Admin::Hotel::ReservationUtils".deconstantize # => "Admin::Hotel"

NOTE: Defined in active_support/core_ext/string/inflections.rb.

parameterize

The method parameterize normalizes its receiver in a way that can be used in pretty URLs.

"John Smith".parameterize # => "john-smith"
"Kurt Gödel".parameterize # => "kurt-godel"

To preserve the case of the string, set the preserve_case argument to true. By default, preserve_case is set to false.

"John Smith".parameterize(preserve_case: true) # => "John-Smith"
"Kurt Gödel".parameterize(preserve_case: true) # => "Kurt-Godel"

To use a custom separator, override the separator argument.

"John Smith".parameterize(separator: "_") # => "john\_smith"
"Kurt Gödel".parameterize(separator: "_") # => "kurt\_godel"

In fact, the result string is wrapped in an instance of ::ActiveSupport::Multibyte::Chars.

NOTE: Defined in active_support/core_ext/string/inflections.rb.

tableize

The method tableize is underscore followed by pluralize.

"Person".tableize      # => "people"
"Invoice".tableize     # => "invoices"
"InvoiceLine".tableize # => "invoice_lines"

As a rule of thumb, tableize returns the table name that corresponds to a given model for simple cases. The actual implementation in Active Record is not straight tableize indeed, because it also demodulizes the class name and checks a few options that may affect the returned string.

NOTE: Defined in active_support/core_ext/string/inflections.rb.

classify

The method classify is the inverse of tableize. It gives you the class name corresponding to a table name:

"people".classify        # => "Person"
"invoices".classify      # => "Invoice"
"invoice_lines".classify # => "InvoiceLine"

The method understands qualified table names:

"highrise_production.companies".classify # => "Company"

Note that classify returns a class name as a string. You can get the actual class object invoking constantize on it, explained next.

NOTE: Defined in active_support/core_ext/string/inflections.rb.

constantize

The method constantize resolves the constant reference expression in its receiver:

"Integer".constantize # => Integer

module M
  X = 1
end
"M::X".constantize # => 1

If the string evaluates to no known constant, or its content is not even a valid constant name, constantize raises NameError.

Constant name resolution by constantize starts always at the top-level Object even if there is no leading "::".

X = :in_Object
module M
  X = :in_M

  X                 # => :in_M
  "::X".constantize # => :in_Object
  "X".constantize   # => :in_Object (!)
end

So, it is in general not equivalent to what Ruby would do in the same spot, had a real constant be evaluated.

Mailer test cases obtain the mailer being tested from the name of the test class using constantize:

# action_mailer/test_case.rb
def determine_default_mailer(name)
  name.sub(/Test$/, '').constantize
rescue NameError => e
  raise NonInferrableMailerError.new(name)
end

NOTE: Defined in active_support/core_ext/string/inflections.rb.

humanize

The method humanize tweaks an attribute name for display to end users.

Specifically performs these transformations:

• Applies human inflection rules to the argument.
• Deletes leading underscores, if any.
• Removes a "_id" suffix if present.
• Replaces underscores with spaces, if any.
• Downcases all words except acronyms.
• Capitalizes the first word.

The capitalization of the first word can be turned off by setting the :capitalize option to false (default is true).

"name".humanize                         # => "Name"
"author_id".humanize                    # => "Author"
"author_id".humanize(capitalize: false) # => "author"
"comments_count".humanize               # => "Comments count"
"_id".humanize                          # => "Id"

If "SSL" was defined to be an acronym:

'ssl_error'.humanize # => "SSL error"

The helper method full_messages uses humanize as a fallback to include attribute names:

def full_messages
  map { |attribute, message| full_message(attribute, message) }
end

def full_message
  #...
  attr_name = attribute.to_s.tr('.', '_').humanize
  attr_name = @base.class.human_attribute_name(attribute, default: attr_name)
  #...
end

NOTE: Defined in active_support/core_ext/string/inflections.rb.

foreign_key

The method foreign_key gives a foreign key column name from a class name. To do so it demodulizes, underscores, and adds "_id":

"User".foreign_key           # => "user_id"
"InvoiceLine".foreign_key    # => "invoice_line_id"
"Admin::Session".foreign_key # => "session_id"

Pass a false argument if you do not want the underscore in "_id":

"User".foreign_key(false) # => "userid"

Associations use this method to infer foreign keys, for example has_one and has_many do this:

# active_record/associations.rb
foreign_key = options[:foreign_key] || reflection.active_record.name.foreign_key

NOTE: Defined in active_support/core_ext/string/inflections.rb.

Conversions

to_date, to_time, to_datetime

The methods to_date, to_time, and to_datetime are basically convenience wrappers around Date._parse:

"2010-07-27".to_date              # => Tue, 27 Jul 2010
"2010-07-27 23:37:00".to_time     # => 2010-07-27 23:37:00 +0200
"2010-07-27 23:37:00".to_datetime # => Tue, 27 Jul 2010 23:37:00 +0000

to_time receives an optional argument :utc or :local, to indicate which time zone you want the time in:

"2010-07-27 23:42:00".to_time(:utc)   # => 2010-07-27 23:42:00 UTC
"2010-07-27 23:42:00".to_time(:local) # => 2010-07-27 23:42:00 +0200

Default is :local.

Please refer to the documentation of Date._parse for further details.

INFO: The three of them return nil for blank receivers.

NOTE: Defined in active_support/core_ext/string/conversions.rb.

Extensions to Numeric

Bytes

All numbers respond to these methods:

bytes
kilobytes
megabytes
gigabytes
terabytes
petabytes
exabytes

They return the corresponding amount of bytes, using a conversion factor of 1024:

2.kilobytes   # => 2048
3.megabytes   # => 3145728
3.5.gigabytes # => 3758096384
-4.exabytes   # => -4611686018427387904

Singular forms are aliased so you are able to say:

1.megabyte # => 1048576

NOTE: Defined in active_support/core_ext/numeric/bytes.rb.

Time

Enables the use of time calculations and declarations, like 45.minutes + 2.hours + 4.weeks.

These methods use Time#advance for precise date calculations when using from_now, ago, etc. as well as adding or subtracting their results from a Time object. For example:

# equivalent to Time.current.advance(months: 1)
1.month.from_now

# equivalent to Time.current.advance(weeks: 2)
2.weeks.from_now

# equivalent to Time.current.advance(months: 4, weeks: 5)
(4.months + 5.weeks).from_now

WARNING. For other durations please refer to the time extensions to Integer.

NOTE: Defined in active_support/core_ext/numeric/time.rb.

Formatting

Enables the formatting of numbers in a variety of ways.

Produce a string representation of a number as a telephone number:

5551234.to_s(:phone)
# => 555-1234
1235551234.to_s(:phone)
# => 123-555-1234
1235551234.to_s(:phone, area_code: true)
# => (123) 555-1234
1235551234.to_s(:phone, delimiter: " ")
# => 123 555 1234
1235551234.to_s(:phone, area_code: true, extension: 555)
# => (123) 555-1234 x 555
1235551234.to_s(:phone, country_code: 1)
# => +1-123-555-1234

Produce a string representation of a number as currency:

1234567890.50.to_s(:currency)                 # => $1,234,567,890.50
1234567890.506.to_s(:currency)                # => $1,234,567,890.51
1234567890.506.to_s(:currency, precision: 3)  # => $1,234,567,890.506

Produce a string representation of a number as a percentage:

100.to_s(:percentage)
# => 100.000%
100.to_s(:percentage, precision: 0)
# => 100%
1000.to_s(:percentage, delimiter: '.', separator: ',')
# => 1.000,000%
302.24398923423.to_s(:percentage, precision: 5)
# => 302.24399%

Produce a string representation of a number in delimited form:

12345678.to_s(:delimited)                     # => 12,345,678
12345678.05.to_s(:delimited)                  # => 12,345,678.05
12345678.to_s(:delimited, delimiter: ".")     # => 12.345.678
12345678.to_s(:delimited, delimiter: ",")     # => 12,345,678
12345678.05.to_s(:delimited, separator: " ")  # => 12,345,678 05

Produce a string representation of a number rounded to a precision:

111.2345.to_s(:rounded)                     # => 111.235
111.2345.to_s(:rounded, precision: 2)       # => 111.23
13.to_s(:rounded, precision: 5)             # => 13.00000
389.32314.to_s(:rounded, precision: 0)      # => 389
111.2345.to_s(:rounded, significant: true)  # => 111

Produce a string representation of a number as a human-readable number of bytes:

123.to_s(:human_size)                  # => 123 Bytes
1234.to_s(:human_size)                 # => 1.21 KB
12345.to_s(:human_size)                # => 12.1 KB
1234567.to_s(:human_size)              # => 1.18 MB
1234567890.to_s(:human_size)           # => 1.15 GB
1234567890123.to_s(:human_size)        # => 1.12 TB
1234567890123456.to_s(:human_size)     # => 1.1 PB
1234567890123456789.to_s(:human_size)  # => 1.07 EB

Produce a string representation of a number in human-readable words:

123.to_s(:human)               # => "123"
1234.to_s(:human)              # => "1.23 Thousand"
12345.to_s(:human)             # => "12.3 Thousand"
1234567.to_s(:human)           # => "1.23 Million"
1234567890.to_s(:human)        # => "1.23 Billion"
1234567890123.to_s(:human)     # => "1.23 Trillion"
1234567890123456.to_s(:human)  # => "1.23 Quadrillion"

NOTE: Defined in active_support/core_ext/numeric/conversions.rb.

Extensions to Integer

multiple_of?

The method multiple_of? tests whether an integer is multiple of the argument:

2.multiple_of?(1) # => true
1.multiple_of?(2) # => false

NOTE: Defined in active_support/core_ext/integer/multiple.rb.

ordinal

The method ordinal returns the ordinal suffix string corresponding to the receiver integer:

1.ordinal    # => "st"
2.ordinal    # => "nd"
53.ordinal   # => "rd"
2009.ordinal # => "th"
-21.ordinal  # => "st"
-134.ordinal # => "th"

NOTE: Defined in active_support/core_ext/integer/inflections.rb.

ordinalize

The method ordinalize returns the ordinal string corresponding to the receiver integer. In comparison, note that the ordinal method returns only the suffix string.

1.ordinalize    # => "1st"
2.ordinalize    # => "2nd"
53.ordinalize   # => "53rd"
2009.ordinalize # => "2009th"
-21.ordinalize  # => "-21st"
-134.ordinalize # => "-134th"

NOTE: Defined in active_support/core_ext/integer/inflections.rb.

Time

Enables the use of time calculations and declarations, like 4.months + 5.years.

These methods use Time#advance for precise date calculations when using from_now, ago, etc. as well as adding or subtracting their results from a Time object. For example:

# equivalent to Time.current.advance(months: 1)
1.month.from_now

# equivalent to Time.current.advance(years: 2)
2.years.from_now

# equivalent to Time.current.advance(months: 4, years: 5)
(4.months + 5.years).from_now

WARNING. For other durations please refer to the time extensions to Numeric.

NOTE: Defined in active_support/core_ext/integer/time.rb.

Extensions to BigDecimal

to_s

The method to_s provides a default specifier of "F". This means that a simple call to to_s will result in floating point representation instead of engineering notation:

BigDecimal(5.00, 6).to_s       # => "5.0"

and that symbol specifiers are also supported:

BigDecimal(5.00, 6).to_s(:db)  # => "5.0"

Engineering notation is still supported:

BigDecimal(5.00, 6).to_s("e")  # => "0.5E1"

Extensions to Enumerable

sum

The method sum adds the elements of an enumerable:

[1, 2, 3].sum # => 6
(1..100).sum  # => 5050

Addition only assumes the elements respond to +:

[[1, 2], [2, 3], [3, 4]].sum    # => [1, 2, 2, 3, 3, 4]
%w(foo bar baz).sum             # => "foobarbaz"
{a: 1, b: 2, c: 3}.sum          # => [:b, 2, :c, 3, :a, 1]

The sum of an empty collection is zero by default, but this is customizable:

[].sum    # => 0
[].sum(1) # => 1

If a block is given, sum becomes an iterator that yields the elements of the collection and sums the returned values:

(1..5).sum {|n| n * 2 } # => 30
[2, 4, 6, 8, 10].sum    # => 30

The sum of an empty receiver can be customized in this form as well:

[].sum(1) {|n| n**3} # => 1

NOTE: Defined in active_support/core_ext/enumerable.rb.

index_by

The method index_by generates a hash with the elements of an enumerable indexed by some key.

It iterates through the collection and passes each element to a block. The element will be keyed by the value returned by the block:

invoices.index_by(&:number)
# => {'2009-032' => <Invoice ...>, '2009-008' => <Invoice ...>, ...}

WARNING. Keys should normally be unique. If the block returns the same value for different elements no collection is built for that key. The last item will win.

NOTE: Defined in active_support/core_ext/enumerable.rb.

many?

The method many? is shorthand for collection.size > 1:

<% if pages.many? %>
  <%= pagination_links %>
<% end %>

If an optional block is given, many? only takes into account those elements that return true:

@see_more = videos.many? {|video| video.category == params[:category]}

NOTE: Defined in active_support/core_ext/enumerable.rb.

exclude?

The predicate exclude? tests whether a given object does not belong to the collection. It is the negation of the built-in include?:

to_visit << node if visited.exclude?(node)

NOTE: Defined in active_support/core_ext/enumerable.rb.

without

The method without returns a copy of an enumerable with the specified elements removed:

["David", "Rafael", "Aaron", "Todd"].without("Aaron", "Todd") # => ["David", "Rafael"]

NOTE: Defined in active_support/core_ext/enumerable.rb.

pluck

The method pluck returns an array based on the given key:

[{ name: "David" }, { name: "Rafael" }, { name: "Aaron" }].pluck(:name) # => ["David", "Rafael", "Aaron"]

NOTE: Defined in active_support/core_ext/enumerable.rb.

Extensions to Array

Accessing

Active Support augments the API of arrays to ease certain ways of accessing them. For example, to returns the subarray of elements up to the one at the passed index:

%w(a b c d).to(2) # => ["a", "b", "c"]
[].to(7)          # => []

Similarly, from returns the tail from the element at the passed index to the end. If the index is greater than the length of the array, it returns an empty array.

%w(a b c d).from(2)  # => ["c", "d"]
%w(a b c d).from(10) # => []
[].from(0)           # => []

The methods second, third, fourth, and fifth return the corresponding element, as do second_to_last and third_to_last (first and last are built-in). Thanks to social wisdom and positive constructiveness all around, forty_two is also available.

%w(a b c d).third # => "c"
%w(a b c d).fifth # => nil

NOTE: Defined in active_support/core_ext/array/access.rb.

Adding Elements

prepend

This method is an alias of Array#unshift.

%w(a b c d).prepend('e')  # => ["e", "a", "b", "c", "d"]
[].prepend(10)            # => [10]

NOTE: Defined in active_support/core_ext/array/prepend_and_append.rb.

append

This method is an alias of Array#<<.

%w(a b c d).append('e')  # => ["a", "b", "c", "d", "e"]
[].append([1,2])         # => [[1, 2]]

NOTE: Defined in active_support/core_ext/array/prepend_and_append.rb.

Options Extraction

When the last argument in a method call is a hash, except perhaps for a &block argument, Ruby allows you to omit the brackets:

User.exists?(email: params[:email])

That syntactic sugar is used a lot in Rails to avoid positional arguments where there would be too many, offering instead interfaces that emulate named parameters. In particular it is very idiomatic to use a trailing hash for options.

If a method expects a variable number of arguments and uses * in its declaration, however, such an options hash ends up being an item of the array of arguments, where it loses its role.

In those cases, you may give an options hash a distinguished treatment with extract_options!. This method checks the type of the last item of an array. If it is a hash it pops it and returns it, otherwise it returns an empty hash.

Let's see for example the definition of the caches_action controller macro:

def caches_action(*actions)
  return unless cache_configured?
  options = actions.extract_options!
  #...
end

This method receives an arbitrary number of action names, and an optional hash of options as last argument. With the call to extract_options! you obtain the options hash and remove it from actions in a simple and explicit way.

NOTE: Defined in active_support/core_ext/array/extract_options.rb.

Conversions

to_sentence

The method to_sentence turns an array into a string containing a sentence that enumerates its items:

%w().to_sentence                # => ""
%w(Earth).to_sentence           # => "Earth"
%w(Earth Wind).to_sentence      # => "Earth and Wind"
%w(Earth Wind Fire).to_sentence # => "Earth, Wind, and Fire"

This method accepts three options:

• :two_words_connector: What is used for arrays of length 2. Default is " and ".
• :words_connector: What is used to join the elements of arrays with 3 or more elements, except for the last two. Default is ", ".
• :last_word_connector: What is used to join the last items of an array with 3 or more elements. Default is ", and ".

The defaults for these options can be localized, their keys are:

Option	I18n key
:two_words_connector	support.array.two_words_connector
:words_connector	support.array.words_connector
:last_word_connector	support.array.last_word_connector

NOTE: Defined in active_support/core_ext/array/conversions.rb.

to_formatted_s

The method to_formatted_s acts like to_s by default.

If the array contains items that respond to id, however, the symbol :db may be passed as argument. That's typically used with collections of Active Record objects. Returned strings are:

[].to_formatted_s(:db)            # => "null"
[user].to_formatted_s(:db)        # => "8456"
invoice.lines.to_formatted_s(:db) # => "23,567,556,12"

Integers in the example above are supposed to come from the respective calls to id.

NOTE: Defined in active_support/core_ext/array/conversions.rb.

to_xml

The method to_xml returns a string containing an XML representation of its receiver:

Contributor.limit(2).order(:rank).to_xml
# =>
# <?xml version="1.0" encoding="UTF-8"?>
# <contributors type="array">
#   <contributor>
#     <id type="integer">4356</id>
#     <name>Jeremy Kemper</name>
#     <rank type="integer">1</rank>
#     <url-id>jeremy-kemper</url-id>
#   </contributor>
#   <contributor>
#     <id type="integer">4404</id>
#     <name>David Heinemeier Hansson</name>
#     <rank type="integer">2</rank>
#     <url-id>david-heinemeier-hansson</url-id>
#   </contributor>
# </contributors>

To do so it sends to_xml to every item in turn, and collects the results under a root node. All items must respond to to_xml, an exception is raised otherwise.

By default, the name of the root element is the underscored and dasherized plural of the name of the class of the first item, provided the rest of elements belong to that type (checked with is_a?) and they are not hashes. In the example above that's "contributors".

If there's any element that does not belong to the type of the first one the root node becomes "objects":

[Contributor.first, Commit.first].to_xml
# =>
# <?xml version="1.0" encoding="UTF-8"?>
# <objects type="array">
#   <object>
#     <id type="integer">4583</id>
#     <name>Aaron Batalion</name>
#     <rank type="integer">53</rank>
#     <url-id>aaron-batalion</url-id>
#   </object>
#   <object>
#     <author>Joshua Peek</author>
#     <authored-timestamp type="datetime">2009-09-02T16:44:36Z</authored-timestamp>
#     <branch>origin/master</branch>
#     <committed-timestamp type="datetime">2009-09-02T16:44:36Z</committed-timestamp>
#     <committer>Joshua Peek</committer>
#     <git-show nil="true"></git-show>
#     <id type="integer">190316</id>
#     <imported-from-svn type="boolean">false</imported-from-svn>
#     <message>Kill AMo observing wrap_with_notifications since ARes was only using it</message>
#     <sha1>723a47bfb3708f968821bc969a9a3fc873a3ed58</sha1>
#   </object>
# </objects>

If the receiver is an array of hashes the root element is by default also "objects":

[{a: 1, b: 2}, {c: 3}].to_xml
# =>
# <?xml version="1.0" encoding="UTF-8"?>
# <objects type="array">
#   <object>
#     <b type="integer">2</b>
#     <a type="integer">1</a>
#   </object>
#   <object>
#     <c type="integer">3</c>
#   </object>
# </objects>

WARNING. If the collection is empty the root element is by default "nil-classes". That's a gotcha, for example the root element of the list of contributors above would not be "contributors" if the collection was empty, but "nil-classes". You may use the :root option to ensure a consistent root element.

The name of children nodes is by default the name of the root node singularized. In the examples above we've seen "contributor" and "object". The option :children allows you to set these node names.

The default XML builder is a fresh instance of Builder::XmlMarkup. You can configure your own builder via the :builder option. The method also accepts options like :dasherize and friends, they are forwarded to the builder:

Contributor.limit(2).order(:rank).to_xml(skip_types: true)
# =>
# <?xml version="1.0" encoding="UTF-8"?>
# <contributors>
#   <contributor>
#     <id>4356</id>
#     <name>Jeremy Kemper</name>
#     <rank>1</rank>
#     <url-id>jeremy-kemper</url-id>
#   </contributor>
#   <contributor>
#     <id>4404</id>
#     <name>David Heinemeier Hansson</name>
#     <rank>2</rank>
#     <url-id>david-heinemeier-hansson</url-id>
#   </contributor>
# </contributors>

NOTE: Defined in active_support/core_ext/array/conversions.rb.

Wrapping

The method Array.wrap wraps its argument in an array unless it is already an array (or array-like).

Specifically:

• If the argument is nil an empty array is returned.
• Otherwise, if the argument responds to to_ary it is invoked, and if the value of to_ary is not nil, it is returned.
• Otherwise, an array with the argument as its single element is returned.

Array.wrap(nil)       # => []
Array.wrap([1, 2, 3]) # => [1, 2, 3]
Array.wrap(0)         # => [0]

This method is similar in purpose to Kernel#Array, but there are some differences:

• If the argument responds to to_ary the method is invoked. Kernel#Array moves on to try to_a if the returned value is nil, but Array.wrap returns an array with the argument as its single element right away.
• If the returned value from to_ary is neither nil nor an Array object, Kernel#Array raises an exception, while Array.wrap does not, it just returns the value.
• It does not call to_a on the argument, if the argument does not respond to to_ary it returns an array with the argument as its single element.

The last point is particularly worth comparing for some enumerables:

Array.wrap(foo: :bar) # => [{:foo=>:bar}]
Array(foo: :bar)      # => [[:foo, :bar]]

There's also a related idiom that uses the splat operator:

[*object]

which in Ruby 1.8 returns [nil] for nil, and calls to Array(object) otherwise. (Please if you know the exact behavior in 1.9 contact fxn.)

Thus, in this case the behavior is different for nil, and the differences with Kernel#Array explained above apply to the rest of objects.

NOTE: Defined in active_support/core_ext/array/wrap.rb.

Duplicating

The method Array#deep_dup duplicates itself and all objects inside recursively with Active Support method Object#deep_dup. It works like Array#map with sending deep_dup method to each object inside.

array = [1, [2, 3]]
dup = array.deep_dup
dup[1][2] = 4
array[1][2] == nil   # => true

NOTE: Defined in active_support/core_ext/object/deep_dup.rb.

Grouping

in_groups_of(number, fill_with = nil)

The method in_groups_of splits an array into consecutive groups of a certain size. It returns an array with the groups:

[1, 2, 3].in_groups_of(2) # => [[1, 2], [3, nil]]

or yields them in turn if a block is passed:

<% sample.in_groups_of(3) do |a, b, c| %>
  <tr>
    <td><%= a %></td>
    <td><%= b %></td>
    <td><%= c %></td>
  </tr>
<% end %>

The first example shows in_groups_of fills the last group with as many nil elements as needed to have the requested size. You can change this padding value using the second optional argument:

[1, 2, 3].in_groups_of(2, 0) # => [[1, 2], [3, 0]]

And you can tell the method not to fill the last group passing false:

[1, 2, 3].in_groups_of(2, false) # => [[1, 2], [3]]

As a consequence false can't be a used as a padding value.

NOTE: Defined in active_support/core_ext/array/grouping.rb.

in_groups(number, fill_with = nil)

The method in_groups splits an array into a certain number of groups. The method returns an array with the groups:

%w(1 2 3 4 5 6 7).in_groups(3)
# => [["1", "2", "3"], ["4", "5", nil], ["6", "7", nil]]

or yields them in turn if a block is passed:

%w(1 2 3 4 5 6 7).in_groups(3) {|group| p group}
["1", "2", "3"]
["4", "5", nil]
["6", "7", nil]

The examples above show that in_groups fills some groups with a trailing nil element as needed. A group can get at most one of these extra elements, the rightmost one if any. And the groups that have them are always the last ones.

You can change this padding value using the second optional argument:

%w(1 2 3 4 5 6 7).in_groups(3, "0")
# => [["1", "2", "3"], ["4", "5", "0"], ["6", "7", "0"]]

And you can tell the method not to fill the smaller groups passing false:

%w(1 2 3 4 5 6 7).in_groups(3, false)
# => [["1", "2", "3"], ["4", "5"], ["6", "7"]]

As a consequence false can't be a used as a padding value.

NOTE: Defined in active_support/core_ext/array/grouping.rb.

split(value = nil)

The method split divides an array by a separator and returns the resulting chunks.

If a block is passed the separators are those elements of the array for which the block returns true:

(-5..5).to_a.split { |i| i.multiple_of?(4) }
# => [[-5], [-3, -2, -1], [1, 2, 3], [5]]

Otherwise, the value received as argument, which defaults to nil, is the separator:

[0, 1, -5, 1, 1, "foo", "bar"].split(1)
# => [[0], [-5], [], ["foo", "bar"]]

TIP: Observe in the previous example that consecutive separators result in empty arrays.

NOTE: Defined in active_support/core_ext/array/grouping.rb.

Extensions to Hash

Conversions

to_xml

The method to_xml returns a string containing an XML representation of its receiver:

{"foo" => 1, "bar" => 2}.to_xml
# =>
# <?xml version="1.0" encoding="UTF-8"?>
# <hash>
#   <foo type="integer">1</foo>
#   <bar type="integer">2</bar>
# </hash>

To do so, the method loops over the pairs and builds nodes that depend on the values. Given a pair key, value:

• If value is a hash there's a recursive call with key as :root.

• If value is an array there's a recursive call with key as :root, and key singularized as :children.

• If value is a callable object it must expect one or two arguments. Depending on the arity, the callable is invoked with the options hash as first argument with key as :root, and key singularized as second argument. Its return value becomes a new node.

• If value responds to to_xml the method is invoked with key as :root.

• Otherwise, a node with key as tag is created with a string representation of value as text node. If value is nil an attribute "nil" set to "true" is added. Unless the option :skip_types exists and is true, an attribute "type" is added as well according to the following mapping:

XML_TYPE_NAMES = {
  "Symbol"     => "symbol",
  "Integer"    => "integer",
  "BigDecimal" => "decimal",
  "Float"      => "float",
  "TrueClass"  => "boolean",
  "FalseClass" => "boolean",
  "Date"       => "date",
  "DateTime"   => "datetime",
  "Time"       => "datetime"
}

By default the root node is "hash", but that's configurable via the :root option.

The default XML builder is a fresh instance of Builder::XmlMarkup. You can configure your own builder with the :builder option. The method also accepts options like :dasherize and friends, they are forwarded to the builder.

NOTE: Defined in active_support/core_ext/hash/conversions.rb.

Merging

Ruby has a built-in method Hash#merge that merges two hashes:

{a: 1, b: 1}.merge(a: 0, c: 2)
# => {:a=>0, :b=>1, :c=>2}

Active Support defines a few more ways of merging hashes that may be convenient.

reverse_merge and reverse_merge!

In case of collision the key in the hash of the argument wins in merge. You can support option hashes with default values in a compact way with this idiom:

options = {length: 30, omission: "..."}.merge(options)

Active Support defines reverse_merge in case you prefer this alternative notation:

options = options.reverse_merge(length: 30, omission: "...")

And a bang version reverse_merge! that performs the merge in place:

options.reverse_merge!(length: 30, omission: "...")

WARNING. Take into account that reverse_merge! may change the hash in the caller, which may or may not be a good idea.

NOTE: Defined in active_support/core_ext/hash/reverse_merge.rb.

reverse_update

The method reverse_update is an alias for reverse_merge!, explained above.

WARNING. Note that reverse_update has no bang.

NOTE: Defined in active_support/core_ext/hash/reverse_merge.rb.

deep_merge and deep_merge!

As you can see in the previous example if a key is found in both hashes the value in the one in the argument wins.

Active Support defines Hash#deep_merge. In a deep merge, if a key is found in both hashes and their values are hashes in turn, then their merge becomes the value in the resulting hash:

{a: {b: 1}}.deep_merge(a: {c: 2})
# => {:a=>{:b=>1, :c=>2}}

The method deep_merge! performs a deep merge in place.

NOTE: Defined in active_support/core_ext/hash/deep_merge.rb.

Deep duplicating

The method Hash#deep_dup duplicates itself and all keys and values inside recursively with Active Support method Object#deep_dup. It works like Enumerator#each_with_object with sending deep_dup method to each pair inside.

hash = { a: 1, b: { c: 2, d: [3, 4] } }

dup = hash.deep_dup
dup[:b][:e] = 5
dup[:b][:d] << 5

hash[:b][:e] == nil      # => true
hash[:b][:d] == [3, 4]   # => true

NOTE: Defined in active_support/core_ext/object/deep_dup.rb.

Working with Keys

except and except!

The method except returns a hash with the keys in the argument list removed, if present:

{a: 1, b: 2}.except(:a) # => {:b=>2}

If the receiver responds to convert_key, the method is called on each of the arguments. This allows except to play nice with hashes with indifferent access for instance:

{a: 1}.with_indifferent_access.except(:a)  # => {}
{a: 1}.with_indifferent_access.except("a") # => {}

There's also the bang variant except! that removes keys in the very receiver.

NOTE: Defined in active_support/core_ext/hash/except.rb.

transform_keys and transform_keys!

The method transform_keys accepts a block and returns a hash that has applied the block operations to each of the keys in the receiver:

{nil => nil, 1 => 1, a: :a}.transform_keys { |key| key.to_s.upcase }
# => {"" => nil, "1" => 1, "A" => :a}

In case of key collision, one of the values will be chosen. The chosen value may not always be the same given the same hash:

{"a" => 1, a: 2}.transform_keys { |key| key.to_s.upcase }
# The result could either be
# => {"A"=>2}
# or
# => {"A"=>1}

This method may be useful for example to build specialized conversions. For instance stringify_keys and symbolize_keys use transform_keys to perform their key conversions:

def stringify_keys
  transform_keys { |key| key.to_s }
end
#...
def symbolize_keys
  transform_keys { |key| key.to_sym rescue key }
end

There's also the bang variant transform_keys! that applies the block operations to keys in the very receiver.

Besides that, one can use deep_transform_keys and deep_transform_keys! to perform the block operation on all the keys in the given hash and all the hashes nested into it. An example of the result is:

{nil => nil, 1 => 1, nested: {a: 3, 5 => 5}}.deep_transform_keys { |key| key.to_s.upcase }
# => {""=>nil, "1"=>1, "NESTED"=>{"A"=>3, "5"=>5}}

NOTE: Defined in active_support/core_ext/hash/keys.rb.

stringify_keys and stringify_keys!

The method stringify_keys returns a hash that has a stringified version of the keys in the receiver. It does so by sending to_s to them:

{nil => nil, 1 => 1, a: :a}.stringify_keys
# => {"" => nil, "1" => 1, "a" => :a}

In case of key collision, one of the values will be chosen. The chosen value may not always be the same given the same hash:

{"a" => 1, a: 2}.stringify_keys
# The result could either be
# => {"a"=>2}
# or
# => {"a"=>1}

This method may be useful for example to easily accept both symbols and strings as options. For instance ::ActionView::Helpers::FormHelper defines:

def to_check_box_tag(options = {}, checked_value = "1", unchecked_value = "0")
  options = options.stringify_keys
  options["type"] = "checkbox"
  #...
end

The second line can safely access the "type" key, and let the user to pass either :type or "type".

There's also the bang variant stringify_keys! that stringifies keys in the very receiver.

Besides that, one can use deep_stringify_keys and deep_stringify_keys! to stringify all the keys in the given hash and all the hashes nested into it. An example of the result is:

{nil => nil, 1 => 1, nested: {a: 3, 5 => 5}}.deep_stringify_keys
# => {""=>nil, "1"=>1, "nested"=>{"a"=>3, "5"=>5}}

NOTE: Defined in active_support/core_ext/hash/keys.rb.

symbolize_keys and symbolize_keys!

The method symbolize_keys returns a hash that has a symbolized version of the keys in the receiver, where possible. It does so by sending to_sym to them:

{nil => nil, 1 => 1, "a" => "a"}.symbolize_keys
# => {nil=>nil, 1=>1, :a=>"a"}

WARNING. Note in the previous example only one key was symbolized.

In case of key collision, one of the values will be chosen. The chosen value may not always be the same given the same hash:

{"a" => 1, a: 2}.symbolize_keys
# The result could either be
# => {:a=>2}
# or
# => {:a=>1}

This method may be useful for example to easily accept both symbols and strings as options. For instance ActionController::UrlRewriter defines

def rewrite_path(options)
  options = options.symbolize_keys
  options.update(options[:params].symbolize_keys) if options[:params]
  #...
end

The second line can safely access the :params key, and let the user to pass either :params or "params".

There's also the bang variant symbolize_keys! that symbolizes keys in the very receiver.

Besides that, one can use deep_symbolize_keys and deep_symbolize_keys! to symbolize all the keys in the given hash and all the hashes nested into it. An example of the result is:

{nil => nil, 1 => 1, "nested" => {"a" => 3, 5 => 5}}.deep_symbolize_keys
# => {nil=>nil, 1=>1, nested:{a:3, 5=>5}}

NOTE: Defined in active_support/core_ext/hash/keys.rb.

to_options and to_options!

The methods to_options and to_options! are respectively aliases of symbolize_keys and symbolize_keys!.

NOTE: Defined in active_support/core_ext/hash/keys.rb.

assert_valid_keys

The method assert_valid_keys receives an arbitrary number of arguments, and checks whether the receiver has any key outside that white list. If it does ArgumentError is raised.

{a: 1}.assert_valid_keys(:a)  # passes
{a: 1}.assert_valid_keys("a") # ArgumentError

Active Record does not accept unknown options when building associations, for example. It implements that control via assert_valid_keys.

NOTE: Defined in active_support/core_ext/hash/keys.rb.

Working with Values

transform_values && transform_values!

The method transform_values accepts a block and returns a hash that has applied the block operations to each of the values in the receiver.

{ nil => nil, 1 => 1, :x => :a }.transform_values { |value| value.to_s.upcase }
# => {nil=>"", 1=>"1", :x=>"A"}

There's also the bang variant transform_values! that applies the block operations to values in the very receiver.

NOTE: Defined in active_support/core_ext/hash/transform_values.rb.

Slicing

Ruby has built-in support for taking slices out of strings and arrays. Active Support extends slicing to hashes:

{a: 1, b: 2, c: 3}.slice(:a, :c)
# => {:a=>1, :c=>3}

{a: 1, b: 2, c: 3}.slice(:b, :X)
# => {:b=>2} # non-existing keys are ignored

If the receiver responds to convert_key keys are normalized:

{a: 1, b: 2}.with_indifferent_access.slice("a")
# => {:a=>1}

NOTE. Slicing may come in handy for sanitizing option hashes with a white list of keys.

There's also slice! which in addition to perform a slice in place returns what's removed:

hash = {a: 1, b: 2}
rest = hash.slice!(:a) # => {:b=>2}
hash                   # => {:a=>1}

NOTE: Defined in active_support/core_ext/hash/slice.rb.

Extracting

The method extract! removes and returns the key/value pairs matching the given keys.

hash = {a: 1, b: 2}
rest = hash.extract!(:a) # => {:a=>1}
hash                     # => {:b=>2}

The method extract! returns the same subclass of Hash, that the receiver is.

hash = {a: 1, b: 2}.with_indifferent_access
rest = hash.extract!(:a).class
# => ActiveSupport::HashWithIndifferentAccess

NOTE: Defined in active_support/core_ext/hash/slice.rb.

Indifferent Access

The method with_indifferent_access returns an ::ActiveSupport::HashWithIndifferentAccess out of its receiver:

{a: 1}.with_indifferent_access["a"] # => 1

NOTE: Defined in active_support/core_ext/hash/indifferent_access.rb.

Compacting

The methods compact and compact! return a Hash without items with nil value.

{a: 1, b: 2, c: nil}.compact # => {a: 1, b: 2}

NOTE: Defined in active_support/core_ext/hash/compact.rb.

Extensions to Regexp

multiline?

The method multiline? says whether a regexp has the /m flag set, that is, whether the dot matches newlines.

%r{.}.multiline?  # => false
%r{.}m.multiline? # => true

Regexp.new('.').multiline?                    # => false
Regexp.new('.', Regexp::MULTILINE).multiline? # => true

Rails uses this method in a single place, also in the routing code. Multiline regexps are disallowed for route requirements and this flag eases enforcing that constraint.

def assign_route_options(segments, defaults, requirements)
  #...
  if requirement.multiline?
    raise ArgumentError, "Regexp multiline option not allowed in routing requirements: #{requirement.inspect}"
  end
  #...
end

NOTE: Defined in active_support/core_ext/regexp.rb.

match?

Rails implements Regexp#match? for Ruby versions prior to 2.4:

/oo/.match?('foo')    # => true
/oo/.match?('bar')    # => false
/oo/.match?('foo', 1) # => true

The backport has the same interface and lack of side-effects in the caller like not setting $1 and friends, but it does not have the speed benefits. Its purpose is to be able to write 2.4 compatible code. Rails itself uses this predicate internally for example.

Active Support defines Regexp#match? only if not present, so code running under 2.4 or later does run the original one and gets the performance boost.

Extensions to Range

to_s

Active Support extends the method Range#to_s so that it understands an optional format argument. As of this writing the only supported non-default format is :db:

(Date.today..Date.tomorrow).to_s
# => "2009-10-25..2009-10-26"

(Date.today..Date.tomorrow).to_s(:db)
# => "BETWEEN '2009-10-25' AND '2009-10-26'"

As the example depicts, the :db format generates a BETWEEN SQL clause. That is used by Active Record in its support for range values in conditions.

NOTE: Defined in active_support/core_ext/range/conversions.rb.

===, include?, and cover?

The methods Range#===, Range#include?, and Range#cover? say whether some value falls between the ends of a given instance:

(2..3).include?(Math::E) # => true

Active Support extends these methods so that the argument may be another range in turn. In that case we test whether the ends of the argument range belong to the receiver themselves:

(1..10) === (3..7)  # => true
(1..10) === (0..7)  # => false
(1..10) === (3..11) # => false
(1...9) === (3..9)  # => false

(1..10).include?(3..7)  # => true
(1..10).include?(0..7)  # => false
(1..10).include?(3..11) # => false
(1...9).include?(3..9)  # => false

(1..10).cover?(3..7)  # => true
(1..10).cover?(0..7)  # => false
(1..10).cover?(3..11) # => false
(1...9).cover?(3..9)  # => false

NOTE: Defined in active_support/core_ext/range/compare_range.rb.

overlaps?

The method Range#overlaps? says whether any two given ranges have non-void intersection:

(1..10).overlaps?(7..11)  # => true
(1..10).overlaps?(0..7)   # => true
(1..10).overlaps?(11..27) # => false

NOTE: Defined in active_support/core_ext/range/overlaps.rb.

Extensions to Date

Calculations

NOTE: All the following methods are defined in active_support/core_ext/date/calculations.rb.

yesterday
tomorrow
beginning_of_week (at_beginning_of_week)
end_of_week (at_end_of_week)
monday
sunday
weeks_ago
prev_week (last_week)
next_week
months_ago
months_since
beginning_of_month (at_beginning_of_month)
end_of_month (at_end_of_month)
last_month
beginning_of_quarter (at_beginning_of_quarter)
end_of_quarter (at_end_of_quarter)
beginning_of_year (at_beginning_of_year)
end_of_year (at_end_of_year)
years_ago
years_since
last_year
on_weekday?
on_weekend?

INFO: The following calculation methods have edge cases in October 1582, since days 5..14 just do not exist. This guide does not document their behavior around those days for brevity, but it is enough to say that they do what you would expect. That is, Date.new(1582, 10, 4).tomorrow returns Date.new(1582, 10, 15) and so on. Please check test/core_ext/date_ext_test.rb in the Active Support test suite for expected behavior.

Date.current

Active Support defines Date.current to be today in the current time zone. That's like Date.today, except that it honors the user time zone, if defined. It also defines Date.yesterday and Date.tomorrow, and the instance predicates past?, today?, future?, on_weekday? and on_weekend?, all of them relative to Date.current.

When making Date comparisons using methods which honor the user time zone, make sure to use Date.current and not Date.today. There are cases where the user time zone might be in the future compared to the system time zone, which Date.today uses by default. This means Date.today may equal Date.yesterday.

Named dates

beginning_of_week, end_of_week

The methods beginning_of_week and end_of_week return the dates for the beginning and end of the week, respectively. Weeks are assumed to start on Monday, but that can be changed passing an argument, setting thread local Date.beginning_of_week or config.beginning_of_week.

d = Date.new(2010, 5, 8)     # => Sat, 08 May 2010
d.beginning_of_week          # => Mon, 03 May 2010
d.beginning_of_week(:sunday) # => Sun, 02 May 2010
d.end_of_week                # => Sun, 09 May 2010
d.end_of_week(:sunday)       # => Sat, 08 May 2010

beginning_of_week is aliased to at_beginning_of_week and end_of_week is aliased to at_end_of_week.

monday, sunday

The methods monday and sunday return the dates for the previous Monday and next Sunday, respectively.

d = Date.new(2010, 5, 8)     # => Sat, 08 May 2010
d.monday                     # => Mon, 03 May 2010
d.sunday                     # => Sun, 09 May 2010

d = Date.new(2012, 9, 10)    # => Mon, 10 Sep 2012
d.monday                     # => Mon, 10 Sep 2012

d = Date.new(2012, 9, 16)    # => Sun, 16 Sep 2012
d.sunday                     # => Sun, 16 Sep 2012

prev_week, next_week

The method next_week receives a symbol with a day name in English (default is the thread local Date.beginning_of_week, or config.beginning_of_week, or :monday) and it returns the date corresponding to that day.

d = Date.new(2010, 5, 9) # => Sun, 09 May 2010
d.next_week              # => Mon, 10 May 2010
d.next_week(:saturday)   # => Sat, 15 May 2010

The method prev_week is analogous:

d.prev_week              # => Mon, 26 Apr 2010
d.prev_week(:saturday)   # => Sat, 01 May 2010
d.prev_week(:friday)     # => Fri, 30 Apr 2010

prev_week is aliased to last_week.

Both next_week and prev_week work as expected when Date.beginning_of_week or config.beginning_of_week are set.

beginning_of_month, end_of_month

The methods beginning_of_month and end_of_month return the dates for the beginning and end of the month:

d = Date.new(2010, 5, 9) # => Sun, 09 May 2010
d.beginning_of_month     # => Sat, 01 May 2010
d.end_of_month           # => Mon, 31 May 2010

beginning_of_month is aliased to at_beginning_of_month, and end_of_month is aliased to at_end_of_month.

beginning_of_quarter, end_of_quarter

The methods beginning_of_quarter and end_of_quarter return the dates for the beginning and end of the quarter of the receiver's calendar year:

d = Date.new(2010, 5, 9) # => Sun, 09 May 2010
d.beginning_of_quarter   # => Thu, 01 Apr 2010
d.end_of_quarter         # => Wed, 30 Jun 2010

beginning_of_quarter is aliased to at_beginning_of_quarter, and end_of_quarter is aliased to at_end_of_quarter.

beginning_of_year, end_of_year

The methods beginning_of_year and end_of_year return the dates for the beginning and end of the year:

d = Date.new(2010, 5, 9) # => Sun, 09 May 2010
d.beginning_of_year      # => Fri, 01 Jan 2010
d.end_of_year            # => Fri, 31 Dec 2010

beginning_of_year is aliased to at_beginning_of_year, and end_of_year is aliased to at_end_of_year.

Other Date Computations

years_ago, years_since

The method years_ago receives a number of years and returns the same date those many years ago:

date = Date.new(2010, 6, 7)
date.years_ago(10) # => Wed, 07 Jun 2000

years_since moves forward in time:

date = Date.new(2010, 6, 7)
date.years_since(10) # => Sun, 07 Jun 2020

If such a day does not exist, the last day of the corresponding month is returned:

Date.new(2012, 2, 29).years_ago(3)     # => Sat, 28 Feb 2009
Date.new(2012, 2, 29).years_since(3)   # => Sat, 28 Feb 2015

last_year is short-hand for #years_ago(1).

months_ago, months_since

The methods months_ago and months_since work analogously for months:

Date.new(2010, 4, 30).months_ago(2)   # => Sun, 28 Feb 2010
Date.new(2010, 4, 30).months_since(2) # => Wed, 30 Jun 2010

If such a day does not exist, the last day of the corresponding month is returned:

Date.new(2010, 4, 30).months_ago(2)    # => Sun, 28 Feb 2010
Date.new(2009, 12, 31).months_since(2) # => Sun, 28 Feb 2010

last_month is short-hand for #months_ago(1).

weeks_ago

The method weeks_ago works analogously for weeks:

Date.new(2010, 5, 24).weeks_ago(1)    # => Mon, 17 May 2010
Date.new(2010, 5, 24).weeks_ago(2)    # => Mon, 10 May 2010

advance

The most generic way to jump to other days is advance. This method receives a hash with keys :years, :months, :weeks, :days, and returns a date advanced as much as the present keys indicate:

date = Date.new(2010, 6, 6)
date.advance(years: 1, weeks: 2)  # => Mon, 20 Jun 2011
date.advance(months: 2, days: -2) # => Wed, 04 Aug 2010

Note in the previous example that increments may be negative.

To perform the computation the method first increments years, then months, then weeks, and finally days. This order is important towards the end of months. Say for example we are at the end of February of 2010, and we want to move one month and one day forward.

The method advance advances first one month, and then one day, the result is:

Date.new(2010, 2, 28).advance(months: 1, days: 1)
# => Sun, 29 Mar 2010

While if it did it the other way around the result would be different:

Date.new(2010, 2, 28).advance(days: 1).advance(months: 1)
# => Thu, 01 Apr 2010

Changing Components

The method change allows you to get a new date which is the same as the receiver except for the given year, month, or day:

Date.new(2010, 12, 23).change(year: 2011, month: 11)
# => Wed, 23 Nov 2011

This method is not tolerant to non-existing dates, if the change is invalid ArgumentError is raised:

Date.new(2010, 1, 31).change(month: 2)
# => ArgumentError: invalid date

Durations

Durations can be added to and subtracted from dates:

d = Date.current
# => Mon, 09 Aug 2010
d + 1.year
# => Tue, 09 Aug 2011
d - 3.hours
# => Sun, 08 Aug 2010 21:00:00 UTC +00:00

They translate to calls to since or advance. For example here we get the correct jump in the calendar reform:

Date.new(1582, 10, 4) + 1.day
# => Fri, 15 Oct 1582

Timestamps

INFO: The following methods return a Time object if possible, otherwise a DateTime. If set, they honor the user time zone.

beginning_of_day, end_of_day

The method beginning_of_day returns a timestamp at the beginning of the day (00:00:00):

date = Date.new(2010, 6, 7)
date.beginning_of_day # => Mon Jun 07 00:00:00 +0200 2010

The method end_of_day returns a timestamp at the end of the day (23:59:59):

date = Date.new(2010, 6, 7)
date.end_of_day # => Mon Jun 07 23:59:59 +0200 2010

beginning_of_day is aliased to at_beginning_of_day, midnight, at_midnight.

beginning_of_hour, end_of_hour

The method beginning_of_hour returns a timestamp at the beginning of the hour (hh:00:00):

date = DateTime.new(2010, 6, 7, 19, 55, 25)
date.beginning_of_hour # => Mon Jun 07 19:00:00 +0200 2010

The method end_of_hour returns a timestamp at the end of the hour (hh:59:59):

date = DateTime.new(2010, 6, 7, 19, 55, 25)
date.end_of_hour # => Mon Jun 07 19:59:59 +0200 2010

beginning_of_hour is aliased to at_beginning_of_hour.

beginning_of_minute, end_of_minute

The method beginning_of_minute returns a timestamp at the beginning of the minute (hh:mm:00):

date = DateTime.new(2010, 6, 7, 19, 55, 25)
date.beginning_of_minute # => Mon Jun 07 19:55:00 +0200 2010

The method end_of_minute returns a timestamp at the end of the minute (hh:mm:59):

date = DateTime.new(2010, 6, 7, 19, 55, 25)
date.end_of_minute # => Mon Jun 07 19:55:59 +0200 2010

beginning_of_minute is aliased to at_beginning_of_minute.

INFO: beginning_of_hour, end_of_hour, beginning_of_minute and end_of_minute are implemented for Time and DateTime but not Date as it does not make sense to request the beginning or end of an hour or minute on a Date instance.

ago, since

The method ago receives a number of seconds as argument and returns a timestamp those many seconds ago from midnight:

date = Date.current # => Fri, 11 Jun 2010
date.ago(1)         # => Thu, 10 Jun 2010 23:59:59 EDT -04:00

Similarly, since moves forward:

date = Date.current # => Fri, 11 Jun 2010
date.since(1)       # => Fri, 11 Jun 2010 00:00:01 EDT -04:00

Other Time Computations

Conversions

Extensions to DateTime

WARNING: DateTime is not aware of DST rules and so some of these methods have edge cases when a DST change is going on. For example seconds_since_midnight might not return the real amount in such a day.

Calculations

NOTE: All the following methods are defined in active_support/core_ext/date_time/calculations.rb.

The class DateTime is a subclass of Date so by loading active_support/core_ext/date/calculations.rb you inherit these methods and their aliases, except that they will always return datetimes.

The following methods are reimplemented so you do not need to load active_support/core_ext/date/calculations.rb for these ones:

beginning_of_day (midnight, at_midnight, at_beginning_of_day)
end_of_day
ago
since (in)

On the other hand, advance and change are also defined and support more options, they are documented below.

The following methods are only implemented in active_support/core_ext/date_time/calculations.rb as they only make sense when used with a DateTime instance:

beginning_of_hour (at_beginning_of_hour)
end_of_hour

Named Datetimes

DateTime.current

Active Support defines DateTime.current to be like Time.now.to_datetime, except that it honors the user time zone, if defined. It also defines DateTime.yesterday and DateTime.tomorrow, and the instance predicates past?, and future? relative to DateTime.current.

Other Extensions

seconds_since_midnight

The method seconds_since_midnight returns the number of seconds since midnight:

now = DateTime.current     # => Mon, 07 Jun 2010 20:26:36 +0000
now.seconds_since_midnight # => 73596

utc

The method utc gives you the same datetime in the receiver expressed in UTC.

now = DateTime.current # => Mon, 07 Jun 2010 19:27:52 -0400
now.utc                # => Mon, 07 Jun 2010 23:27:52 +0000

This method is also aliased as getutc.

utc?

The predicate utc? says whether the receiver has UTC as its time zone:

now = DateTime.now # => Mon, 07 Jun 2010 19:30:47 -0400
now.utc?           # => false
now.utc.utc?       # => true

advance

The most generic way to jump to another datetime is advance. This method receives a hash with keys :years, :months, :weeks, :days, :hours, :minutes, and :seconds, and returns a datetime advanced as much as the present keys indicate.

d = DateTime.current
# => Thu, 05 Aug 2010 11:33:31 +0000
d.advance(years: 1, months: 1, days: 1, hours: 1, minutes: 1, seconds: 1)
# => Tue, 06 Sep 2011 12:34:32 +0000

This method first computes the destination date passing :years, :months, :weeks, and :days to Date#advance documented above. After that, it adjusts the time calling since with the number of seconds to advance. This order is relevant, a different ordering would give different datetimes in some edge-cases. The example in Date#advance applies, and we can extend it to show order relevance related to the time bits.

If we first move the date bits (that have also a relative order of processing, as documented before), and then the time bits we get for example the following computation:

d = DateTime.new(2010, 2, 28, 23, 59, 59)
# => Sun, 28 Feb 2010 23:59:59 +0000
d.advance(months: 1, seconds: 1)
# => Mon, 29 Mar 2010 00:00:00 +0000

but if we computed them the other way around, the result would be different:

d.advance(seconds: 1).advance(months: 1)
# => Thu, 01 Apr 2010 00:00:00 +0000

WARNING: Since DateTime is not DST-aware you can end up in a non-existing point in time with no warning or error telling you so.

Changing Components

The method change allows you to get a new datetime which is the same as the receiver except for the given options, which may include :year, :month, :day, :hour, :min, :sec, :offset, :start:

now = DateTime.current
# => Tue, 08 Jun 2010 01:56:22 +0000
now.change(year: 2011, offset: Rational(-6, 24))
# => Wed, 08 Jun 2011 01:56:22 -0600

If hours are zeroed, then minutes and seconds are too (unless they have given values):

now.change(hour: 0)
# => Tue, 08 Jun 2010 00:00:00 +0000

Similarly, if minutes are zeroed, then seconds are too (unless it has given a value):

now.change(min: 0)
# => Tue, 08 Jun 2010 01:00:00 +0000

This method is not tolerant to non-existing dates, if the change is invalid ArgumentError is raised:

DateTime.current.change(month: 2, day: 30)
# => ArgumentError: invalid date

Durations

Durations can be added to and subtracted from datetimes:

now = DateTime.current
# => Mon, 09 Aug 2010 23:15:17 +0000
now + 1.year
# => Tue, 09 Aug 2011 23:15:17 +0000
now - 1.week
# => Mon, 02 Aug 2010 23:15:17 +0000

They translate to calls to since or advance. For example here we get the correct jump in the calendar reform:

DateTime.new(1582, 10, 4, 23) + 1.hour
# => Fri, 15 Oct 1582 00:00:00 +0000

Extensions to Time

Calculations

NOTE: All the following methods are defined in active_support/core_ext/time/calculations.rb.

past?
today?
future?
yesterday
tomorrow
seconds_since_midnight
change
advance
ago
since (in)
prev_day
next_day
beginning_of_day (midnight, at_midnight, at_beginning_of_day)
end_of_day
beginning_of_hour (at_beginning_of_hour)
end_of_hour
beginning_of_week (at_beginning_of_week)
end_of_week (at_end_of_week)
monday
sunday
weeks_ago
prev_week (last_week)
next_week
months_ago
months_since
beginning_of_month (at_beginning_of_month)
end_of_month (at_end_of_month)
prev_month
next_month
last_month
beginning_of_quarter (at_beginning_of_quarter)
end_of_quarter (at_end_of_quarter)
beginning_of_year (at_beginning_of_year)
end_of_year (at_end_of_year)
years_ago
years_since
prev_year
last_year
next_year
on_weekday?
on_weekend?

They are analogous. Please refer to their documentation above and take into account the following differences:

• change accepts an additional :usec option.
• Time understands DST, so you get correct DST calculations as in

Time.zone_default
# => #<ActiveSupport::TimeZone:0x7f73654d4f38 @utc_offset=nil, @name="Madrid", ...>

# In Barcelona, 2010/03/28 02:00 +0100 becomes 2010/03/28 03:00 +0200 due to DST.
t = Time.local(2010, 3, 28, 1, 59, 59)
# => Sun Mar 28 01:59:59 +0100 2010
t.advance(seconds: 1)
# => Sun Mar 28 03:00:00 +0200 2010

• If since or ago jump to a time that can't be expressed with Time a DateTime object is returned instead.

Time.current

Active Support defines Time.current to be today in the current time zone. That's like Time.now, except that it honors the user time zone, if defined. It also defines the instance predicates past?, today?, and future?, all of them relative to Time.current.

When making Time comparisons using methods which honor the user time zone, make sure to use Time.current instead of Time.now. There are cases where the user time zone might be in the future compared to the system time zone, which Time.now uses by default. This means Time.now.to_date may equal Date.yesterday.

all_day, all_week, all_month, all_quarter and all_year

The method all_day returns a range representing the whole day of the current time.

now = Time.current
# => Mon, 09 Aug 2010 23:20:05 UTC +00:00
now.all_day
# => Mon, 09 Aug 2010 00:00:00 UTC +00:00..Mon, 09 Aug 2010 23:59:59 UTC +00:00

Analogously, all_week, all_month, all_quarter and all_year all serve the purpose of generating time ranges.

now = Time.current
# => Mon, 09 Aug 2010 23:20:05 UTC +00:00
now.all_week
# => Mon, 09 Aug 2010 00:00:00 UTC +00:00..Sun, 15 Aug 2010 23:59:59 UTC +00:00
now.all_week(:sunday)
# => Sun, 16 Sep 2012 00:00:00 UTC +00:00..Sat, 22 Sep 2012 23:59:59 UTC +00:00
now.all_month
# => Sat, 01 Aug 2010 00:00:00 UTC +00:00..Tue, 31 Aug 2010 23:59:59 UTC +00:00
now.all_quarter
# => Thu, 01 Jul 2010 00:00:00 UTC +00:00..Thu, 30 Sep 2010 23:59:59 UTC +00:00
now.all_year
# => Fri, 01 Jan 2010 00:00:00 UTC +00:00..Fri, 31 Dec 2010 23:59:59 UTC +00:00

prev_day, next_day

In Ruby 1.9 prev_day and next_day return the date in the last or next day:

d = Date.new(2010, 5, 8) # => Sat, 08 May 2010
d.prev_day               # => Fri, 07 May 2010
d.next_day               # => Sun, 09 May 2010

prev_month, next_month

In Ruby 1.9 prev_month and next_month return the date with the same day in the last or next month:

d = Date.new(2010, 5, 8) # => Sat, 08 May 2010
d.prev_month             # => Thu, 08 Apr 2010
d.next_month             # => Tue, 08 Jun 2010

If such a day does not exist, the last day of the corresponding month is returned:

Date.new(2000, 5, 31).prev_month # => Sun, 30 Apr 2000
Date.new(2000, 3, 31).prev_month # => Tue, 29 Feb 2000
Date.new(2000, 5, 31).next_month # => Fri, 30 Jun 2000
Date.new(2000, 1, 31).next_month # => Tue, 29 Feb 2000

prev_year, next_year

In Ruby 1.9 prev_year and next_year return a date with the same day/month in the last or next year:

d = Date.new(2010, 5, 8) # => Sat, 08 May 2010
d.prev_year              # => Fri, 08 May 2009
d.next_year              # => Sun, 08 May 2011

If date is the 29th of February of a leap year, you obtain the 28th:

d = Date.new(2000, 2, 29) # => Tue, 29 Feb 2000
d.prev_year               # => Sun, 28 Feb 1999
d.next_year               # => Wed, 28 Feb 2001

prev_quarter, next_quarter

prev_quarter and next_quarter return the date with the same day in the previous or next quarter:

t = Time.local(2010, 5, 8) # => 2010-05-08 00:00:00 +0300
t.prev_quarter             # => 2010-02-08 00:00:00 +0200
t.next_quarter             # => 2010-08-08 00:00:00 +0300

If such a day does not exist, the last day of the corresponding month is returned:

Time.local(2000, 7, 31).prev_quarter  # => 2000-04-30 00:00:00 +0300
Time.local(2000, 5, 31).prev_quarter  # => 2000-02-29 00:00:00 +0200
Time.local(2000, 10, 31).prev_quarter # => 2000-07-31 00:00:00 +0300
Time.local(2000, 11, 31).next_quarter # => 2001-03-01 00:00:00 +0200

prev_quarter is aliased to last_quarter.

Time Constructors

Active Support defines Time.current to be Time.zone.now if there's a user time zone defined, with fallback to Time.now:

Time.zone_default
# => #<ActiveSupport::TimeZone:0x7f73654d4f38 @utc_offset=nil, @name="Madrid", ...>
Time.current
# => Fri, 06 Aug 2010 17:11:58 CEST +02:00

Analogously to DateTime, the predicates past?, and future? are relative to Time.current.

If the time to be constructed lies beyond the range supported by Time in the runtime platform, usecs are discarded and a DateTime object is returned instead.

Durations

Durations can be added to and subtracted from time objects:

now = Time.current
# => Mon, 09 Aug 2010 23:20:05 UTC +00:00
now + 1.year
# => Tue, 09 Aug 2011 23:21:11 UTC +00:00
now - 1.week
# => Mon, 02 Aug 2010 23:21:11 UTC +00:00

They translate to calls to since or advance. For example here we get the correct jump in the calendar reform:

Time.utc(1582, 10, 3) + 5.days
# => Mon Oct 18 00:00:00 UTC 1582

Extensions to File

atomic_write

With the class method File.atomic_write you can write to a file in a way that will prevent any reader from seeing half-written content.

The name of the file is passed as an argument, and the method yields a file handle opened for writing. Once the block is done atomic_write closes the file handle and completes its job.

For example, Action Pack uses this method to write asset cache files like all.css:

File.atomic_write(joined_asset_path) do |cache|
  cache.write(join_asset_file_contents(asset_paths))
end

To accomplish this atomic_write creates a temporary file. That's the file the code in the block actually writes to. On completion, the temporary file is renamed, which is an atomic operation on POSIX systems. If the target file exists atomic_write overwrites it and keeps owners and permissions. However there are a few cases where atomic_write cannot change the file ownership or permissions, this error is caught and skipped over trusting in the user/filesystem to ensure the file is accessible to the processes that need it.

NOTE. Due to the chmod operation atomic_write performs, if the target file has an ACL set on it this ACL will be recalculated/modified.

WARNING. Note you can't append with atomic_write.

The auxiliary file is written in a standard directory for temporary files, but you can pass a directory of your choice as second argument.

NOTE: Defined in active_support/core_ext/file/atomic.rb.

Extensions to Marshal

load

Active Support adds constant autoloading support to load.

For example, the file cache store deserializes this way:

File.open(file_name) { |f| Marshal.load(f) }

If the cached data refers to a constant that is unknown at that point, the autoloading mechanism is triggered and if it succeeds the deserialization is retried transparently.

WARNING. If the argument is an IO it needs to respond to rewind to be able to retry. Regular files respond to rewind.

NOTE: Defined in active_support/core_ext/marshal.rb.

Extensions to NameError

Active Support adds missing_name? to NameError, which tests whether the exception was raised because of the name passed as argument.

The name may be given as a symbol or string. A symbol is tested against the bare constant name, a string is against the fully qualified constant name.

TIP: A symbol can represent a fully qualified constant name as in :"ActiveRecord::Base", so the behavior for symbols is defined for convenience, not because it has to be that way technically.

For example, when an action of ArticlesController is called Rails tries optimistically to use ArticlesHelper. It is OK that the helper module does not exist, so if an exception for that constant name is raised it should be silenced. But it could be the case that articles_helper.rb raises a NameError due to an actual unknown constant. That should be reraised. The method missing_name? provides a way to distinguish both cases:

def default_helper_module!
  module_name = name.sub(/Controller$/, '')
  module_path = module_name.underscore
  helper module_path
rescue LoadError => e
  raise e unless e.is_missing? "helpers/#{module_path}_helper"
rescue NameError => e
  raise e unless e.missing_name? "#{module_name}Helper"
end

NOTE: Defined in active_support/core_ext/name_error.rb.

Extensions to LoadError

Active Support adds is_missing? to LoadError.

Given a path name is_missing? tests whether the exception was raised due to that particular file (except perhaps for the ".rb" extension).

For example, when an action of ArticlesController is called Rails tries to load articles_helper.rb, but that file may not exist. That's fine, the helper module is not mandatory so Rails silences a load error. But it could be the case that the helper module does exist and in turn requires another library that is missing. In that case Rails must reraise the exception. The method is_missing? provides a way to distinguish both cases:

def default_helper_module!
  module_name = name.sub(/Controller$/, '')
  module_path = module_name.underscore
  helper module_path
rescue LoadError => e
  raise e unless e.is_missing? "helpers/#{module_path}_helper"
rescue NameError => e
  raise e unless e.missing_name? "#{module_name}Helper"
end

NOTE: Defined in active_support/core_ext/load_error.rb.