class User
  include Mongoid::Document
  include Mongoid::Timestamps

  field :name, type: String
  has_many :comments
end

class Comment
  include Mongoid::Document
  include Mongoid::Timestamps
 
  field :content, type: String
  belongs_to :user, counter_cache: true
end

user = User.create(name: 'Jeremy')
user.comments_count
#NoMethodError: undefined method `comments_count' for #<User:0x007fd64ac70d08>
#from /Users/jeremy/.rvm/gems/ruby-2.0.0-p0@tabeso/gems/mongoid-3.1.2/lib/mongoid/attributes.rb:318:in `method_missing'
user.comments.count #=> 0
User.update_counters(user.id, comments_count: 0)
user.comments_count #=> 0