MongoID counter_cache hack

gistfile1.rb

#
# Save this in `lib/mongoid/counter_cache.rb`
#
# Then define your relations kinda like that
# 
#   class Comment
#     belongs_to :post, counter_cache: true  # 'comments_count'
#     belongs_to :user, counter_cache: 'my_field_name'
#   end
#
module Mongoid
  module CounterCache
    def self.included(base)
      base.instance_eval do
        def belongs_to(name, options = {}, &block)
          if counter_field = options.delete(:counter_cache)
            counter_cache name.to_s, counter_field == true ? nil : counter_field.to_s
          end

          super name, options, &block
        end
      end
    end

    extend ActiveSupport::Concern

    module ClassMethods
      def counter_cache(name, counter_field=nil)
        counter_field ||= self.name.underscore.pluralize + "_count"

        Object.const_get(name.capitalize.camelcase).instance_eval do
          field counter_field.to_sym, type: Integer, default: 0
          index counter_field.to_sym
        end

        after_create do |document|
          document.send(name).inc(counter_field.to_sym, 1)
        end

        after_destroy do |document|
          document.send(name).inc(counter_field.to_sym, -1)
        end
      end
    end
  end
end