MongoId Counter Cahce

gistfile1.rb

# 
#  lib/mongoid/counter_cache.rb
#  ruby
#  
#  Created by Zhang Alex on 2011-06-17.
#  Copyright 2011 __ZhangHanDong__. All rights reserved.
# 
# ===================================
# class Forum
#   references_many :topics
#   references_many :posts
# end
#
#
# class Topic
#   referenced_in :forum
#   include Mongoid::CounterCache
#   counter_cache name: :forum, inverse_of: :posts
# end
# ===================================

module Mongoid
  module CounterCache
    extend ActiveSupport::Concern

    module ClassMethods
      def counter_cache(metadata)
        counter_name = "#{metadata[:inverse_of]}_count"

        set_callback(:create, :after) do |document|
          relation = document.send(metadata[:name])
          if relation
            relation.inc(counter_name.to_sym, 1) if relation.class.fields.keys.include?(counter_name)
          end
        end

        set_callback(:destroy, :after) do |document|
          relation = document.send(metadata[:name])
          if relation && relation.class.fields.keys.include?(counter_name)
            relation.inc(counter_name.to_sym, -1) 
          end
        end

      end

    end #ClassMethods

  end #CounterCache
end #Mongoid