Active Record mixin to also delegate setters

delegate_assign.rb

# Also delegate setters with Rails.
#
# If you find yourself wanting to use this, please look at nested attributes first.
# http://api.rubyonrails.org/classes/ActiveRecord/NestedAttributes/ClassMethods.html
# When that doesn't fit, you may consider the approach shown here.
#
# Example:
#
#     class User < ActiveRecord::Base
#       include DelegateAssign
#
#       has_one :greeter
#       delegate :hello, to: :greeter, assign: true
#     end
#
#     class Greeter < ActiveRecord::Base
#     end
#
#     u = User.create(name: 'John', hello: 'Howdy')
#     u.hello # => 'Howdy'
#     u.greeter.hello # => 'Howdy'
#
module DelegateAssign
  extend ActiveSupport::Concern

  module ClassMethods
    # @todo validate delegated attributes (and pass through errors if needed)
    # @todo check object is not saved when delegated object has an error
    # @todo warn when +assign+ is used with +to: :class+ (or make sure it works)
    # @todo handle +prefix+ option
    def delegate(*attrs, **options)
      assign = options.delete(:assign)
      all_attrs = attrs

      # delegate writer methods too when asked
      all_attrs += attrs.map{|a| "#{a}=".to_sym} if assign

      super(*all_attrs, **options)

      if assign
        to = options[:to]
        defs = []

        # make sure there is a delegated object before its attribute is set
        attrs.each do |attr|
          defs += [
            "def #{attr}_with_build=(value)",          # def hello_with_build=(value)
            "  self.#{to} ||= build_#{to}",            #   self.greeter ||= build_greeter
            "  self.#{attr}_without_build = value",    #   self.hello_without_build = value
            "end",                                     # end

            "alias_method_chain '#{attr}=', :build",   # alias_method_chain hello=, :build
          ]
        end

        # and make sure changes in delegated methods are stored
        defs += [
          "after_update do",                          # after_update do
          "  #{to}.save if #{to} && #{to}.changed?",  #   greeter.save if greeter && greeter.changed?
          "end"                                       # end
        ]

        file, line = caller.first.split(':', 2)
        line = line.to_i
        module_eval(defs.join(';'), file, line)
      end
    end
  end
end