Mongoid counter cache

forum.rb

class Forum
  include Mongoid::Document
  include Mongoid::Timestamps

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

  has_many_related :posts
end

mongoid_counter_cache.rb

module Mongoid
  module CounterCache
    extend ActiveSupport::Concern

    module ClassMethods
      def counter_cache(options)
        name = options[:name]
        counter_field = options[:field]

        after_create do |document|
          relation = document.send(name)
          relation.collection.update(relation._selector, {'$inc' => {counter_field.to_s => 1}}, {:multi => true})
        end

        after_destroy do |document|
          relation = document.send(name)
          relation.collection.update(relation._selector, {'$inc' => {counter_field.to_s => -1}}, {:multi => true})
        end
      end
    end

  end
end

post.rb

class Post
  include Mongoid::Document
  include Mongoid::Timestamps
  include Mongoid::CounterCache

  counter_cache :name => 'forum', :field => 'posts_count'

  belongs_to_related :forum
end

why do you use :multi => true? For a given Post, it would belong to one Forum, so on create / destroy it would only be updating the counter cache for a singular document in a collection, no? Thanks for sharing, I ended up using a variation of this to handle embedded documents and a counter cache in the parent.

you're right. to be honest, :multi => true is just copy-paste :)

no prob, I've done that thousands of times! I just wanted to make sure I wasn't missing something (still in the shallow end when it comes to Mongo / Mongoid). Thanks for the heads up.

I'm sure I'm missing something simple, but I keep getting an error on "relation._selector": undefined method "_selector". Any thoughts to help a guy out?

Where mongoid_counter_cache.rb is should be placed? /lib ? how to automatically load it in rails 3.1?

I ended up putting it in my concerns directory "app/concerns/mongoid/counter_cache.rb". I haven't used 3.1 yet, but I'm assuming you'd add which ever directory into your load path, which is in "config/application.rb". I have a line that looks like this in mine: "config.autoload_paths += %W(#{config.root}/lib)". However, if you put it in as a concern, anything in the "app" directory will be loaded up automatically anyway.

added mine to config/initializers as I wanted it autoloaded / included for all