Token.user_id = NILL, why? What am I doing wrong?

gistfile1.rb

class User
  include MongoMapper::Document
  
  safe
  
  key :email, String
  key :username, String
  key :first_name, String
  key :last_name, String
  key :encrypted_password, String
  key :password_salt, String
  key :reset_password_token, String
  key :remember_token, String
  key :remember_created_at, Time
  key :sign_in_count, Integer
  key :current_sign_in_at, Time
  key :current_sign_in_ip, String
  key :last_sign_in_at, Time
  key :last_sign_in_ip, String
  timestamps!
  
  many :tokens
  
  attr_accessor :login

  attr_accessible :username, :email, :password, :password_confirmation, :login
  
  # Include default devise modules. Others available are:
  # :token_authenticatable, :encryptable, :confirmable, :lockable, :timeoutable and :omniauthable
  devise :database_authenticatable, :registerable,
         :recoverable, :rememberable, :trackable, :validatable,
         :token_authenticatable, :authentication_keys => [:login]
         
  def self.find_for_database_authentication(conditions)
    login = conditions.delete(:login).downcase
    where('$or' => [{:username => login}, {:email => login}]).first
  end

end

class Token 
  include MongoMapper::Document
  
  key :token, String,   :required => true
  key :date, Time,      :required => true
  key :comment, String
  
  belongs_to :user
end

@token = Token.new(params[:token])
@token.token = "aaaa"
@token.date  = Time.now()
@token.user  = current_user
     
puts current_user.username # => Outputs the username correctly