Swallow Reader extension for Active Record Has One Through Association

swallow_reader.rb

# = Swallow Reader
#
# Adds <tt>:swallow</tt> option to <tt>has_one :through</tt> association builder.
# When this option set to <tt>true</tt> and through association was loaded, target record will 
# be readed directly from parent without executing the join query.
#
# For example:
#
#     class Review < ActiveRecord::Base
#       belongs_to :song
#       has_one :artist, through: :song, swallow: true
#     end
#
#     Review.preload(:song).artist
#
# Without swallow it executes query:
# 
#     SELECT  "artists".* FROM "artists" INNER JOIN "songs" ON "artists"."id" = "songs"."artist_id" WHERE "songs"."id" = $1 LIMIT $2
#
# With <tt>swallow: true</tt>:
# 
#     SELECT  "artists".* FROM "artists" WHERE "artists"."id" = $1 LIMIT $2
#
module SwallowReader
  module BuilderHook
    def valid_options(options)
      valid = super
      valid << :swallow if options[:through]
      valid
    end
  end

  module AssociationMethods
    def reader(force = false)
      read_swallow? ? swallow_reader(force) : super
    end

    def swallow_reader(force = false)
      through_association.reader&.public_send(reflection.name, force)
    end

    private

    def through_association
      @owner.association(options[:through])
    end

    def read_swallow?
      options[:swallow] && !loaded? && through_association.loaded?
    end
  end
end

ActiveSupport.on_load(:active_record) do
  ActiveRecord::Associations::Builder::HasOne.singleton_class.prepend(SwallowReader::BuilderHook)
  ActiveRecord::Associations::HasOneThroughAssociation.prepend(SwallowReader::AssociationMethods)
end