Class: ActiveModel::Validator

  class MyValidator < ActiveModel::Validator
    def validate(record)
      record.errors[:base] << "This is some custom error message"
      record.errors[: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(record, 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[attribute] << 'must be Mr. Mrs. or Dr.' unless ['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, :title => true
  end

Validator may also define a setup instance method which will get called with the class that using that validator as it’s 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.