deleagate_error.rb

module DelegateError
  extend ActiveSupport::Concern
  included do
    class_variable_set(:@@delegate_error_attrs , [])
    after_validation :copy_errors_to_delegated_attribute
  end
  module ClassMethods
    def delegate_errors(s_atr, t_atr)
      attrs = delegate_error_attrs + [{source: s_atr, target: t_atr}]
      class_variable_set(:@@delegate_error_attrs, attrs)
    end

    def delegate_error_attrs
      class_variable_get(:@@delegate_error_attrs)
    end
  end

  private

  def copy_errors_to_delegated_attribute
    self.class.delegate_error_attrs.each do |atr_hash|
      self.errors[atr_hash[:source]].each do |e|
        self.errors[atr_hash[:target]] << e
      end
    end
  end

end