Support:

Rails3.0+/Mongoid2.0.2+/Ruby1.8/1.9

Usage:

class Forum
  references_many :topics
  references_many :posts

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

class Topic
  referenced_in :forum
  include Mongoid::CounterCache
  counter_cache :name => :forum, :inverse_of => posts
end