Mongoid HATBM with string keys

profile.rb

class Profile
  include Mongoid::Document
  # ....
  belongs_to :user
  key :user_id
  alias :to_s :_id

  has_and_belongs_to_many :following, :class_name => 'Profile', :inverse_of => :followers
  has_and_belongs_to_many :followers, :class_name => 'Profile', :inverse_of => :following
  # ....
end

user.rb

class User
  include Mongoid::Document
  # ....
  field :nick
  key :nick
  alias :to_s :_id
  # ....
end