Mongoid and has_secure_password

I am trying to use rails 3.1 authentication using mongoid instead of active model

 class User 
  include Mongoid::Document
  include ActiveModel::SecurePassword
  has_secure_password  
  validates_presence_of :password, :on => :create
  attr_accessor :email, :password, :password_confirmation 
  field :email, :type => String
  field :password_digest, :type => String
 end

the problem is password_digest is not recognized by the bycrypt as in active model example http://railscasts.com/episodes/270-authentication-in-rails-3-1

Thanks