ActiveModel::Validator

Active Model Validator

A simple base class that can be used along with ActiveModel::Validations::ClassMethods.validates_with

class Person
  include ActiveModel::Validations
  validates_with MyValidator
end

class MyValidator < ActiveModel::Validator
  def validate(record)
    if some_complex_logic
      record.errors[:base] = "This record is invalid"
    end
  end

  private
    def some_complex_logic
      # ...
    end
end

Any class that inherits from ActiveModel::Validator must implement a method called validate which accepts a record.

class Person
  include ActiveModel::Validations
  validates_with MyValidator
end

class MyValidator < ActiveModel::Validator
  def validate(record)
    record # => The person instance being validated
    options # => Any non-standard options passed to validates_with
  end
end

To cause a validation error, you must add to the record's errors directly from within the validators message.

class MyValidator < ActiveModel::Validator
  def validate(record)
    record.errors.add :base, "This is some custom error message"
    record.errors.add :first_name, "This is some complex validation"
    # etc...
  end
end

To add behavior to the initialize method, use the following signature:

class MyValidator < ActiveModel::Validator
  def initialize(options)
    super
    @my_custom_field = options[:field_name] || :first_name
  end
end

Note that the validator is initialized only once for the whole application lifecycle, and not on each validation run.

The easiest way to add custom validators for validating individual attributes is with the convenient ActiveModel::EachValidator.

class TitleValidator < ActiveModel::EachValidator
  def validate_each(record, attribute, value)
    record.errors.add attribute, 'must be Mr., Mrs., or Dr.' unless %w(Mr. Mrs. Dr.).include?(value)
  end
end

This can now be used in combination with the validates method (see ActiveModel::Validations::ClassMethods.validates for more on this).

class Person
  include ActiveModel::Validations
  attr_accessor :title

  validates :title, presence: true
end

Validator may also define a setup instance method which will get called with the class that using that validator as its argument. This can be useful when there are prerequisites such as an attr_accessor being present.

class MyValidator < ActiveModel::Validator
  def setup(klass)
    klass.send :attr_accessor, :custom_attribute
  end
end

This setup method is only called when used with validation macros or the class level validates_with method.