Class ActiveModel::Validator < Object

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

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

class TitleValidator < ActiveModel::EachValidator
  def validate_each(record, attribute, value)
    record.errors.add attribute, 'must be Mr. Mrs. or Dr.' unless value.in?(['Mr.', 'Mrs.', 'Dr.'])
  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 for example:

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.