counter cache extension for mongoid orm

mongoid_counter_cache.rb

module Mongoid
  module CounterCache
    extend ActiveSupport::Concern

    module ClassMethods
      # Use Case:
      #
      #  keep a cached count of embedded collections in the parent collection(s)
      #
      #  for example, if a Post embeds many Comments, and there have been 2
      #  comments embedded in the the first post, we can do the following:
      #
      #     >> Post.first.comments_count # 2
      #
      #  which just queries for the post's attribute.
      #
      #  The parent counter field should follow the format:
      #
      #     pluralized embedded collection name + "_count"
      #
      #  Again, using our example from above, Post would have a field of
      #  "comments_count" (type Integer, default of 0). If no
      #  configuration is provided to the counter_cache class method, this
      #  format will be derivated and assumed.
      #
      #  All parents of the embedded document will be checked for a counter
      #  cache to incremement or decrement.
      #
      # Usage:
      #
      #  class Playlist
      #    include Mongoid::Document
      #    include Mongoid::Timestamps
      #    embeds_many :tracks
      #    field :comments_count, type: Integer, default: 0
      #  end
      #
      #  class Track
      #    include Mongoid::Document
      #    include Mongoid::Timestamps
      #    embedded_in :posts
      #    embeds_many :comments, cascade_callbacks: true
      #
      #    field :comments_count, type: Integer, default: 0
      #  end
      #
      #  class Comment
      #    include Mongoid::Document
      #    include Mongoid::Timestamps
      #    include Mongoid::CounterCache # where all the magic happens!
      #    embedded_in :tracks
      #    counter_cache # (optional) :field => "some_field_count", must match parent(s) field definition(s)
      #  end
      #
      #  Then ...
      #    >> # some setup is assumed
      #    >> p = Playlist.first # comments_count = 0
      #    >> t = p.tracks.first # comments_count = 0
      #    >> t.comments << Comment.new
      #    >> p.save # true
      #    >> t.comments_count # 1
      #    >> p.comments_count # 1
      def counter_cache(options = {})
        counter_field = (options[:field] || "#{self.name.downcase.pluralize}_count").to_s

        after_create do |document|
          current_document = document
          while current_document.embedded?
            parent = current_document._parent
            parent.inc(counter_field, 1) if parent.respond_to? counter_field
            current_document = current_document._parent
          end
        end

        after_destroy do |document|
          current_document = document
          while current_document.embedded?
            parent = current_document._parent
            parent.inc(counter_field, -1) if parent.respond_to? counter_field
            current_document = current_document._parent
          end
        end
      end
    end
  end
end