add_child_validations.rb

# In not working!!
# Validations are added, but errors are not in the errors list after valid
module AddChildValidations
  extend ActiveSupport::Concern

  class_methods do
    # Example:
    # class User < ActiveRecord::Base
    #   has_one :address
    #   add_child_validations_for :address do
    #     validates_presence_of :address, :city, :postal_code, :state, :state_code,
    #       :country_code
    #   end
    # end
    def add_child_validations_for(name, opts= {}, &block)
      fail "No block provided for child validations on #{self.class}:#{name}" unless block

      @_child_validations ||= {}
      @_child_validations[name.to_sym] = opts.merge(name: name, block: block)

      self.send(:alias_method, "_old_#{name}=", "#{name}=")
      self.send(:alias_method, "_old_#{name}", name)

      reader_line, reader = __LINE__, "def #{name}
        return @#{name} if @#{name}
        block = _get_child_validations_for('#{name}')[:block]
        _old_#{name}.singleton_class.instance_exec(&block)
        @#{name} ||= _old_#{name}
      end"

      writer_line, writer = __LINE__, "def #{name}=(input)
        @#{name} = nil
        self._old_#{name} = input
      end"
      class_eval reader, __FILE__, reader_line
      class_eval writer, __FILE__, writer_line
    end
  end

  def _child_validations
    self.class.instance_variable_get(:@_child_validations) || {}
  end

  def _get_child_validations_for(name)
    _child_validations[name.to_sym]
  end
end