A Countable Field for Mongoid

image.rb

class Image
  include Mongoid::Document

  field :likes_count, :type => Integer, :default => 0

  has_many :likes

end

like.rb

class Like
  include Mongoid::Document
  include Mongoid::CounterCache

  counter_cache :document => "images", :field => "likes_count"

  belongs_to :image

end

mongoid_counter_cache.rb

module Mongoid
  module CounterCache
    extend ActiveSupport::Concern

    module ClassMethods

      # Public: Allows caching of countable Integer data
      #
      # args -  The Hash args used to select the document and field:
      #         :document - The String document name
      #         :field - The String field name to count
      #
      # Examples
      #
      #   cache_counter :document => "images", :field => "like_count"
      #    => Imaage Object with counter incremented / decremented
      def counter_cache(args)
        model = args[:document]
        field = args[:field]

        after_create do |document|
          relation = document.send(model)
          relation.collection.update(relation._selector, {'$inc' => {field.to_s => 1}})
        end

        after_destroy do |document|
          relation = document.send(model)
          relation.collection.update(relation._selector, {'$inc' => {field.to_s => -1}})
        end
      end
    end

  end
end

Typo: :field => "likes_count" in like.rb