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Active Record Validations

This guide teaches you how to validate the state of objects before they go into the database using Active Record's validations feature.

After reading this guide, you will know:

Validations Overview

Here's an example of a very simple validation:

class Person < ApplicationRecord
  validates :name, presence: true
end

Person.create(name: "John Doe").valid? # => true
Person.create(name: nil).valid? # => false

As you can see, our validation lets us know that our Person is not valid without a name attribute. The second Person will not be persisted to the database.

Before we dig into more details, let's talk about how validations fit into the big picture of your application.

Why Use Validations?

Validations are used to ensure that only valid data is saved into your database. For example, it may be important to your application to ensure that every user provides a valid email address and mailing address. Model-level validations are the best way to ensure that only valid data is saved into your database. They are database agnostic, cannot be bypassed by end users, and are convenient to test and maintain. Rails makes them easy to use, provides built-in helpers for common needs, and allows you to create your own validation methods as well.

There are several other ways to validate data before it is saved into your database, including native database constraints, client-side validations and controller-level validations. Here's a summary of the pros and cons:

Choose these in certain, specific cases. It's the opinion of the Rails team that model-level validations are the most appropriate in most circumstances.

When Does Validation Happen?

There are two kinds of Active Record objects: those that correspond to a row inside your database and those that do not. When you create a fresh object, for example using the new method, that object does not belong to the database yet. Once you call save upon that object it will be saved into the appropriate database table. Active Record uses the new_record? instance method to determine whether an object is already in the database or not. Consider the following simple Active Record class:

class Person < ApplicationRecord
end

We can see how it works by looking at some rails console output:

$ bin/rails console
>> p = Person.new(name: "John Doe")
#=> #<Person id: nil, name: "John Doe", created_at: nil, updated_at: nil>
>> p.new_record?
#=> true
>> p.save
#=> true
>> p.new_record?
#=> false

Creating and saving a new record will send an SQL INSERT operation to the database. Updating an existing record will send an SQL UPDATE operation instead. Validations are typically run before these commands are sent to the database. If any validations fail, the object will be marked as invalid and Active Record will not perform the INSERT or UPDATE operation. This avoids storing an invalid object in the database. You can choose to have specific validations run when an object is created, saved, or updated.

CAUTION: There are many ways to change the state of an object in the database. Some methods will trigger validations, but some will not. This means that it's possible to save an object in the database in an invalid state if you aren't careful.

The following methods trigger validations, and will save the object to the database only if the object is valid:

The bang versions (e.g. save!) raise an exception if the record is invalid. The non-bang versions don't: save and update return false, and create just returns the object.

Skipping Validations

The following methods skip validations, and will save the object to the database regardless of its validity. They should be used with caution.

Note that save also has the ability to skip validations if passed validate: false as an argument. This technique should be used with caution.

valid? and invalid?

Before saving an Active Record object, Rails runs your validations. If these validations produce any errors, Rails does not save the object.

You can also run these validations on your own. valid? triggers your validations and returns true if no errors were found in the object, and false otherwise. As you saw above:

class Person < ApplicationRecord
  validates :name, presence: true
end

Person.create(name: "John Doe").valid? # => true
Person.create(name: nil).valid? # => false

After Active Record has performed validations, any errors found can be accessed through the errors.messages instance method, which returns a collection of errors. By definition, an object is valid if this collection is empty after running validations.

Note that an object instantiated with new will not report errors even if it's technically invalid, because validations are automatically run only when the object is saved, such as with the create or save methods.

class Person < ApplicationRecord
  validates :name, presence: true
end

>> p = Person.new
# => #<Person id: nil, name: nil>
>> p.errors.messages
# => {}

>> p.valid?
# => false
>> p.errors.messages
# => {name:["can't be blank"]}

>> p = Person.create
# => #<Person id: nil, name: nil>
>> p.errors.messages
# => {name:["can't be blank"]}

>> p.save
# => false

>> p.save!
# => ActiveRecord::RecordInvalid: Validation failed: Name can't be blank

>> Person.create!
# => ActiveRecord::RecordInvalid: Validation failed: Name can't be blank

invalid? is simply the inverse of valid?. It triggers your validations, returning true if any errors were found in the object, and false otherwise.

errors[]

To verify whether or not a particular attribute of an object is valid, you can use errors[:attribute]. It returns an array of all the errors for :attribute. If there are no errors on the specified attribute, an empty array is returned.

This method is only useful after validations have been run, because it only inspects the errors collection and does not trigger validations itself. It's different from the ActiveRecord::Base#invalid? method explained above because it doesn't verify the validity of the object as a whole. It only checks to see whether there are errors found on an individual attribute of the object.

class Person < ApplicationRecord
  validates :name, presence: true
end

>> Person.new.errors[:name].any? # => false
>> Person.create.errors[:name].any? # => true

We'll cover validation errors in greater depth in the Working with Validation Errors section.

errors.details

To check which validations failed on an invalid attribute, you can use errors.details[:attribute]. It returns an array of hashes with an :error key to get the symbol of the validator:

class Person < ApplicationRecord
  validates :name, presence: true
end

>> person = Person.new
>> person.valid?
>> person.errors.details[:name] # => [{error: :blank}]

Using details with custom validators is covered in the Working with Validation Errors section.

Validation Helpers

Active Record offers many pre-defined validation helpers that you can use directly inside your class definitions. These helpers provide common validation rules. Every time a validation fails, an error message is added to the object's errors collection, and this message is associated with the attribute being validated.

Each helper accepts an arbitrary number of attribute names, so with a single line of code you can add the same kind of validation to several attributes.

All of them accept the :on and :message options, which define when the validation should be run and what message should be added to the errors collection if it fails, respectively. The :on option takes one of the values :create or :update. There is a default error message for each one of the validation helpers. These messages are used when the :message option isn't specified. Let's take a look at each one of the available helpers.

acceptance

This method validates that a checkbox on the user interface was checked when a form was submitted. This is typically used when the user needs to agree to your application's terms of service, confirm that some text is read, or any similar concept.

class Person < ApplicationRecord
  validates :terms_of_service, acceptance: true
end

This check is performed only if terms_of_service is not nil. The default error message for this helper is "must be accepted". You can also pass custom message via the message option.

class Person < ApplicationRecord
  validates :terms_of_service, acceptance: { message: 'must be abided' }
end

It can also receive an :accept option, which determines the allowed values that will be considered as accepted. It defaults to ['1', true] and can be easily changed.

class Person < ApplicationRecord
  validates :terms_of_service, acceptance: { accept: 'yes' }
  validates :eula, acceptance: { accept: ['TRUE', 'accepted'] }
end

This validation is very specific to web applications and this 'acceptance' does not need to be recorded anywhere in your database. If you don't have a field for it, the helper will just create a virtual attribute. If the field does exist in your database, the accept option must be set to or include true or else the validation will not run.

validates_associated

You should use this helper when your model has associations with other models and they also need to be validated. When you try to save your object, valid? will be called upon each one of the associated objects.

class Library < ApplicationRecord
  has_many :books
  validates_associated :books
end

This validation will work with all of the association types.

CAUTION: Don't use validates_associated on both ends of your associations. They would call each other in an infinite loop.

The default error message for validates_associated is "is invalid". Note that each associated object will contain its own errors collection; errors do not bubble up to the calling model.

confirmation

You should use this helper when you have two text fields that should receive exactly the same content. For example, you may want to confirm an email address or a password. This validation creates a virtual attribute whose name is the name of the field that has to be confirmed with "_confirmation" appended.

class Person < ApplicationRecord
  validates :email, confirmation: true
end

In your view template you could use something like

<%= text_field :person, :email %>
<%= text_field :person, :email_confirmation %>

This check is performed only if email_confirmation is not nil. To require confirmation, make sure to add a presence check for the confirmation attribute (we'll take a look at presence later on in this guide):

class Person < ApplicationRecord
  validates :email, confirmation: true
  validates :email_confirmation, presence: true
end

There is also a :case_sensitive option that you can use to define whether the confirmation constraint will be case sensitive or not. This option defaults to true.

class Person < ApplicationRecord
  validates :email, confirmation: { case_sensitive: false }
end

The default error message for this helper is "doesn't match confirmation".

exclusion

This helper validates that the attributes' values are not included in a given set. In fact, this set can be any enumerable object.

class Account < ApplicationRecord
  validates :subdomain, exclusion: { in: %w(www us ca jp),
    message: "%{value} is reserved." }
end

The exclusion helper has an option :in that receives the set of values that will not be accepted for the validated attributes. The :in option has an alias called :within that you can use for the same purpose, if you'd like to. This example uses the :message option to show how you can include the attribute's value. For full options to the message argument please see the message documentation.

The default error message is "is reserved".

format

This helper validates the attributes' values by testing whether they match a given regular expression, which is specified using the :with option.

class Product < ApplicationRecord
  validates :legacy_code, format: { with: /\A[a-zA-Z]+\z/,
    message: "only allows letters" }
end

Alternatively, you can require that the specified attribute does not match the regular expression by using the :without option.

The default error message is "is invalid".

inclusion

This helper validates that the attributes' values are included in a given set. In fact, this set can be any enumerable object.

class Coffee < ApplicationRecord
  validates :size, inclusion: { in: %w(small medium large),
    message: "%{value} is not a valid size" }
end

The inclusion helper has an option :in that receives the set of values that will be accepted. The :in option has an alias called :within that you can use for the same purpose, if you'd like to. The previous example uses the :message option to show how you can include the attribute's value. For full options please see the message documentation.

The default error message for this helper is "is not included in the list".

length

This helper validates the length of the attributes' values. It provides a variety of options, so you can specify length constraints in different ways:

class Person < ApplicationRecord
  validates :name, length: { minimum: 2 }
  validates :bio, length: { maximum: 500 }
  validates :password, length: { in: 6..20 }
  validates :registration_number, length: { is: 6 }
end

The possible length constraint options are:

The default error messages depend on the type of length validation being performed. You can personalize these messages using the :wrong_length, :too_long, and :too_short options and %{count} as a placeholder for the number corresponding to the length constraint being used. You can still use the :message option to specify an error message.

class Person < ApplicationRecord
  validates :bio, length: { maximum: 1000,
    too_long: "%{count} characters is the maximum allowed" }
end

Note that the default error messages are plural (e.g., "is too short (minimum is %count characters)"). For this reason, when :minimum is 1 you should provide a personalized message or use presence: true instead. When :in or :within have a lower limit of 1, you should either provide a personalized message or call presence prior to length.

numericality

This helper validates that your attributes have only numeric values. By default, it will match an optional sign followed by an integral or floating point number. To specify that only integral numbers are allowed set :only_integer to true.

If you set :only_integer to true, then it will use the

/\A[-]?\d\z/

regular expression to validate the attribute's value. Otherwise, it will try to convert the value to a number using Float.

class Player < ApplicationRecord
  validates :points, numericality: true
  validates :games_played, numericality: { only_integer: true }
end

Besides :only_integer, this helper also accepts the following options to add constraints to acceptable values:

NOTE: By default, numericality doesn't allow nil values. You can use allow_nil: true option to permit it.

The default error message is "is not a number".

presence

This helper validates that the specified attributes are not empty. It uses the blank? method to check if the value is either nil or a blank string, that is, a string that is either empty or consists of whitespace.

class Person < ApplicationRecord
  validates :name, :, :email, presence: true
end

If you want to be sure that an association is present, you'll need to test whether the associated object itself is present, and not the foreign key used to map the association.

class LineItem < ApplicationRecord
  belongs_to :order
  validates :order, presence: true
end

In order to validate associated records whose presence is required, you must specify the :inverse_of option for the association:

class Order < ApplicationRecord
  has_many :line_items, inverse_of: :order
end

If you validate the presence of an object associated via a has_one or has_many relationship, it will check that the object is neither blank? nor marked_for_destruction?.

Since false.blank? is true, if you want to validate the presence of a boolean field you should use one of the following validations:

validates :boolean_field_name, inclusion: { in: [true, false] }
validates :boolean_field_name, exclusion: { in: [nil] }

By using one of these validations, you will ensure the value will NOT be nil which would result in a NULL value in most cases.

absence

This helper validates that the specified attributes are absent. It uses the present? method to check if the value is not either nil or a blank string, that is, a string that is either empty or consists of whitespace.

class Person < ApplicationRecord
  validates :name, :, :email, absence: true
end

If you want to be sure that an association is absent, you'll need to test whether the associated object itself is absent, and not the foreign key used to map the association.

class LineItem < ApplicationRecord
  belongs_to :order
  validates :order, absence: true
end

In order to validate associated records whose absence is required, you must specify the :inverse_of option for the association:

class Order < ApplicationRecord
  has_many :line_items, inverse_of: :order
end

If you validate the absence of an object associated via a has_one or has_many relationship, it will check that the object is neither present? nor marked_for_destruction?.

Since false.present? is false, if you want to validate the absence of a boolean field you should use validates :field_name, exclusion: { in: [true, false] }.

The default error message is "must be blank".

uniqueness

This helper validates that the attribute's value is unique right before the object gets saved. It does not create a uniqueness constraint in the database, so it may happen that two different database connections create two records with the same value for a column that you intend to be unique. To avoid that, you must create a unique index on that column in your database.

class Account < ApplicationRecord
  validates :email, uniqueness: true
end

The validation happens by performing an SQL query into the model's table, searching for an existing record with the same value in that attribute.

There is a :scope option that you can use to specify one or more attributes that are used to limit the uniqueness check:

class Holiday < ApplicationRecord
  validates :name, uniqueness: { scope: :year,
    message: "should happen once per year" }
end

Should you wish to create a database constraint to prevent possible violations of a uniqueness validation using the :scope option, you must create a unique index on both columns in your database. See the MySQL manual for more details about multiple column indexes or the PostgreSQL manual for examples of unique constraints that refer to a group of columns.

There is also a :case_sensitive option that you can use to define whether the uniqueness constraint will be case sensitive or not. This option defaults to true.

class Person < ApplicationRecord
  validates :name, uniqueness: { case_sensitive: false }
end

WARNING. Note that some databases are configured to perform case-insensitive searches anyway.

The default error message is "has already been taken".

validates_with

This helper passes the record to a separate class for validation.

class GoodnessValidator < ActiveModel::Validator
  def validate(record)
    if record.first_name == "Evil"
      record.errors[:base] << "This person is evil"
    end
  end
end

class Person < ApplicationRecord
  validates_with GoodnessValidator
end

NOTE: Errors added to record.errors[:base] relate to the state of the record as a whole, and not to a specific attribute.

The validates_with helper takes a class, or a list of classes to use for validation. There is no default error message for validates_with. You must manually add errors to the record's errors collection in the validator class.

To implement the validate method, you must have a record parameter defined, which is the record to be validated.

Like all other validations, validates_with takes the :if, :unless and :on options. If you pass any other options, it will send those options to the validator class as options:

class GoodnessValidator < ActiveModel::Validator
  def validate(record)
    if options[:fields].any?{|field| record.send(field) == "Evil" }
      record.errors[:base] << "This person is evil"
    end
  end
end

class Person < ApplicationRecord
  validates_with GoodnessValidator, fields: [:first_name, :last_name]
end

Note that the validator will be initialized only once for the whole application life cycle, and not on each validation run, so be careful about using instance variables inside it.

If your validator is complex enough that you want instance variables, you can easily use a plain old Ruby object instead:

class Person < ApplicationRecord
  validate do |person|
    GoodnessValidator.new(person).validate
  end
end

class GoodnessValidator
  def initialize(person)
    @person = person
  end

  def validate
    if some_complex_condition_involving_ivars_and_private_methods?
      @person.errors[:base] << "This person is evil"
    end
  end

  # ...
end

validates_each

This helper validates attributes against a block. It doesn't have a predefined validation function. You should create one using a block, and every attribute passed to validates_each will be tested against it. In the following example, we don't want names and surnames to begin with lower case.

class Person < ApplicationRecord
  validates_each :name, :surname do |record, attr, value|
    record.errors.add(attr, 'must start with upper case') if value =~ /\A[[:lower:]]/
  end
end

The block receives the record, the attribute's name and the attribute's value. You can do anything you like to check for valid data within the block. If your validation fails, you should add an error message to the model, therefore making it invalid.

Common Validation Options

These are common validation options:

:allow_nil

The :allow_nil option skips the validation when the value being validated is nil.

class Coffee < ApplicationRecord
  validates :size, inclusion: { in: %w(small medium large),
    message: "%{value} is not a valid size" }, allow_nil: true
end

For full options to the message argument please see the message documentation.

:allow_blank

The :allow_blank option is similar to the :allow_nil option. This option will let validation pass if the attribute's value is blank?, like nil or an empty string for example.

class Topic < ApplicationRecord
  validates :title, length: { is: 5 }, allow_blank: true
end

Topic.create(title: "").valid?  # => true
Topic.create(title: nil).valid? # => true

:message

As you've already seen, the :message option lets you specify the message that will be added to the errors collection when validation fails. When this option is not used, Active Record will use the respective default error message for each validation helper. The :message option accepts a String or Proc.

A String :message value can optionally contain any/all of %{value}, %{attribute}, and %{model} which will be dynamically replaced when validation fails. This replacement is done using the I18n gem, and the placeholders must match exactly, no spaces are allowed.

A Proc :message value is given two arguments: the object being validated, and a hash with :model, :attribute, and :value key-value pairs.

class Person < ApplicationRecord
  # Hard-coded message
  validates :name, presence: { message: "must be given please" }

  # Message with dynamic attribute value. %{value} will be replaced with
  # the actual value of the attribute. %{attribute} and %{model} also
  # available.
  validates :age, numericality: { message: "%{value} seems wrong" }

  # Proc
  validates :username,
    uniqueness: {
      # object = person object being validated
      # data = { model: "Person", attribute: "Username", value: <username> }
      message: ->(object, data) do
        "Hey #{object.name}!, #{data[:value]} is taken already! Try again #{Time.zone.tomorrow}"
      end
    }
end

:on

The :on option lets you specify when the validation should happen. The default behavior for all the built-in validation helpers is to be run on save (both when you're creating a new record and when you're updating it). If you want to change it, you can use on: :create to run the validation only when a new record is created or on: :update to run the validation only when a record is updated.

class Person < ApplicationRecord
  # it will be possible to update email with a duplicated value
  validates :email, uniqueness: true, on: :create

  # it will be possible to create the record with a non-numerical age
  validates :age, numericality: true, on: :update

  # the default (validates on both create and update)
  validates :name, presence: true
end

You can also use on: to define custom context. Custom contexts need to be triggered explicitly by passing name of the context to valid?, invalid? or save.

class Person < ApplicationRecord
  validates :email, uniqueness: true, on: :
  validates :age, numericality: true, on: :
end

person = Person.new

person.valid?(:account_setup) executes both the validations without saving the model. And person.save(context: :account_setup) validates person in account_setup context before saving. On explicit triggers, model is validated by validations of only that context and validations without context.

Strict Validations

You can also specify validations to be strict and raise ::ActiveModel::StrictValidationFailed when the object is invalid.

class Person < ApplicationRecord
  validates :name, presence: { strict: true }
end

Person.new.valid?  # => ActiveModel::StrictValidationFailed: Name can't be blank

There is also the ability to pass a custom exception to the :strict option.

class Person < ApplicationRecord
  validates :token, presence: true, uniqueness: true, strict: TokenGenerationException
end

Person.new.valid?  # => TokenGenerationException: Token can't be blank

Conditional Validation

Sometimes it will make sense to validate an object only when a given predicate is satisfied. You can do that by using the :if and :unless options, which can take a symbol, a Proc or an Array. You may use the :if option when you want to specify when the validation should happen. If you want to specify when the validation should not happen, then you may use the :unless option.

Using a Symbol with :if and :unless

You can associate the :if and :unless options with a symbol corresponding to the name of a method that will get called right before validation happens. This is the most commonly used option.

class Order < ApplicationRecord
  validates :card_number, presence: true, if: :paid_with_card?

  def paid_with_card?
    payment_type == "card"
  end
end

Using a Proc with :if and :unless

Finally, it's possible to associate :if and :unless with a Proc object which will be called. Using a Proc object gives you the ability to write an inline condition instead of a separate method. This option is best suited for one-liners.

class Account < ApplicationRecord
  validates :password, confirmation: true,
    unless: Proc.new { |a| a.password.blank? }
end

Grouping Conditional validations

Sometimes it is useful to have multiple validations use one condition. It can be easily achieved using with_options.

class User < ApplicationRecord
  with_options if: :is_admin? do |admin|
    admin.validates :password, length: { minimum: 10 }
    admin.validates :email, presence: true
  end
end

All validations inside of the with_options block will have automatically passed the condition if: :is_admin?

Combining Validation Conditions

On the other hand, when multiple conditions define whether or not a validation should happen, an Array can be used. Moreover, you can apply both :if and :unless to the same validation.

class Computer < ApplicationRecord
  validates :mouse, presence: true,
                    if: [Proc.new { |c| c.market.retail? }, :desktop?],
                    unless: Proc.new { |c| c.trackpad.present? }
end

The validation only runs when all the :if conditions and none of the :unless conditions are evaluated to true.

Performing Custom Validations

When the built-in validation helpers are not enough for your needs, you can write your own validators or validation methods as you prefer.

Custom Validators

Custom validators are classes that inherit from ::ActiveModel::Validator. These classes must implement the validate method which takes a record as an argument and performs the validation on it. The custom validator is called using the validates_with method.

class MyValidator < ActiveModel::Validator
  def validate(record)
    unless record.name.starts_with? 'X'
      record.errors[:name] << 'Need a name starting with X please!'
    end
  end
end

class Person
  include ActiveModel::Validations
  validates_with MyValidator
end

The easiest way to add custom validators for validating individual attributes is with the convenient ::ActiveModel::EachValidator. In this case, the custom validator class must implement a validate_each method which takes three arguments: record, attribute, and value. These correspond to the instance, the attribute to be validated, and the value of the attribute in the passed instance.

class EmailValidator < ActiveModel::EachValidator
  def validate_each(record, attribute, value)
    unless value =~ /\A([^@\s])@((?:[-a-z0-9]\.)+[a-z]{2,})\z/i
      record.errors[attribute] << (options[:message] || "is not an email")
    end
  end
end

class Person < ApplicationRecord
  validates :email, presence: true, email: true
end

As shown in the example, you can also combine standard validations with your own custom validators.

Custom Methods

You can also create methods that verify the state of your models and add messages to the errors collection when they are invalid. You must then register these methods by using the validate (API) class method, passing in the symbols for the validation methods' names.

You can pass more than one symbol for each class method and the respective validations will be run in the same order as they were registered.

The valid? method will verify that the errors collection is empty, so your custom validation methods should add errors to it when you wish validation to fail:

class Invoice < ApplicationRecord
  validate :expiration_date_cannot_be_in_the_past,
    :discount_cannot_be_greater_than_total_value

  def expiration_date_cannot_be_in_the_past
    if expiration_date.present? && expiration_date < Date.today
      errors.add(:expiration_date, "can't be in the past")
    end
  end

  def discount_cannot_be_greater_than_total_value
    if discount > total_value
      errors.add(:discount, "can't be greater than total value")
    end
  end
end

By default, such validations will run every time you call valid? or save the object. But it is also possible to control when to run these custom validations by giving an :on option to the validate method, with either: :create or :update.

class Invoice < ApplicationRecord
  validate :active_customer, on: :create

  def active_customer
    errors.add(:customer_id, "is not active") unless customer.active?
  end
end

Working with Validation Errors

In addition to the valid? and invalid? methods covered earlier, Rails provides a number of methods for working with the errors collection and inquiring about the validity of objects.

The following is a list of the most commonly used methods. Please refer to the ::ActiveModel::Errors documentation for a list of all the available methods.

errors

Returns an instance of the class ::ActiveModel::Errors containing all errors. Each key is the attribute name and the value is an array of strings with all errors.

class Person < ApplicationRecord
  validates :name, presence: true, length: { minimum: 3 }
end

person = Person.new
person.valid? # => false
person.errors.messages
 # => {:name=>["can't be blank", "is too short (minimum is 3 characters)"]}

person = Person.new(name: "John Doe")
person.valid? # => true
person.errors.messages # => {}

errors[]

errors[] is used when you want to check the error messages for a specific attribute. It returns an array of strings with all error messages for the given attribute, each string with one error message. If there are no errors related to the attribute, it returns an empty array.

class Person < ApplicationRecord
  validates :name, presence: true, length: { minimum: 3 }
end

person = Person.new(name: "John Doe")
person.valid? # => true
person.errors[:name] # => []

person = Person.new(name: "JD")
person.valid? # => false
person.errors[:name] # => ["is too short (minimum is 3 characters)"]

person = Person.new
person.valid? # => false
person.errors[:name]
 # => ["can't be blank", "is too short (minimum is 3 characters)"]

errors.add

The add method lets you add an error message related to a particular attribute. It takes as arguments the attribute and the error message.

The errors.full_messages method (or its equivalent, errors.to_a) returns the error messages in a user-friendly format, with the capitalized attribute name prepended to each message, as shown in the examples below.

class Person < ApplicationRecord
  def a_method_used_for_validation_purposes
    errors.add(:name, "cannot contain the characters !@#%*()_-+=")
  end
end

person = Person.create(name: "!@#")

person.errors[:name]
 # => ["cannot contain the characters !@#%*()_-+="]

person.errors.full_messages
 # => ["Name cannot contain the characters !@#%*()_-+="]

An equivalent to errors#add is to use << to append a message to the errors.messages array for an attribute:

class Person < ApplicationRecord
  def a_method_used_for_validation_purposes
    errors.messages[:name] << "cannot contain the characters !@#%*()_-+="
  end
end

person = Person.create(name: "!@#")

person.errors[:name]
 # => ["cannot contain the characters !@#%*()_-+="]

person.errors.to_a
 # => ["Name cannot contain the characters !@#%*()_-+="]

errors.details

You can specify a validator type to the returned error details hash using the errors.add method.

class Person < ApplicationRecord
  def a_method_used_for_validation_purposes
    errors.add(:name, :invalid_characters)
  end
end

person = Person.create(name: "!@#")

person.errors.details[:name]
# => [{error: :invalid_characters}]

To improve the error details to contain the unallowed characters set for instance, you can pass additional keys to errors.add.

class Person < ApplicationRecord
  def a_method_used_for_validation_purposes
    errors.add(:name, :invalid_characters, not_allowed: "!@#%*()_-+=")
  end
end

person = Person.create(name: "!@#")

person.errors.details[:name]
# => [{error: :invalid_characters, not_allowed: "!@#%*()_-+="}]

All built in Rails validators populate the details hash with the corresponding validator type.

errors[:base]

You can add error messages that are related to the object's state as a whole, instead of being related to a specific attribute. You can use this method when you want to say that the object is invalid, no matter the values of its attributes. Since errors[:base] is an array, you can simply add a string to it and it will be used as an error message.

class Person < ApplicationRecord
  def a_method_used_for_validation_purposes
    errors[:base] << "This person is invalid because ..."
  end
end

errors.clear

The clear method is used when you intentionally want to clear all the messages in the errors collection. Of course, calling errors.clear upon an invalid object won't actually make it valid: the errors collection will now be empty, but the next time you call valid? or any method that tries to save this object to the database, the validations will run again. If any of the validations fail, the errors collection will be filled again.

class Person < ApplicationRecord
  validates :name, presence: true, length: { minimum: 3 }
end

person = Person.new
person.valid? # => false
person.errors[:name]
 # => ["can't be blank", "is too short (minimum is 3 characters)"]

person.errors.clear
person.errors.empty? # => true

person.save # => false

person.errors[:name]
# => ["can't be blank", "is too short (minimum is 3 characters)"]

errors.size

The size method returns the total number of error messages for the object.

class Person < ApplicationRecord
  validates :name, presence: true, length: { minimum: 3 }
end

person = Person.new
person.valid? # => false
person.errors.size # => 2

person = Person.new(name: "Andrea", email: "andrea@example.com")
person.valid? # => true
person.errors.size # => 0

Displaying Validation Errors in Views

Once you've created a model and added validations, if that model is created via a web form, you probably want to display an error message when one of the validations fail.

Because every application handles this kind of thing differently, Rails does not include any view helpers to help you generate these messages directly. However, due to the rich number of methods Rails gives you to interact with validations in general, it's fairly easy to build your own. In addition, when generating a scaffold, Rails will put some ERB into the _form.html.erb that it generates that displays the full list of errors on that model.

Assuming we have a model that's been saved in an instance variable named @article, it looks like this:

<% if @article.errors.any? %>
  <div id="error_explanation">
    <h2><%= pluralize(@article.errors.count, "error") %> prohibited this article from being saved:</h2>

    <ul>
    <% @article.errors.full_messages.each do |msg| %>
      <li><%= msg %></li>
    <% end %>
    </ul>
  </div>
<% end %>

Furthermore, if you use the Rails form helpers to generate your forms, when a validation error occurs on a field, it will generate an extra <div> around the entry.

<div class="field_with_errors">
 <input id="article_title" name="article[title]" size="30" type="text" value="">
</div>

You can then style this div however you'd like. The default scaffold that Rails generates, for example, adds this CSS rule:

.field_with_errors {
  padding: 2px;
  background-color: red;
  display: table;
}

This means that any field with an error ends up with a 2 pixel red border.