to_param not working in model

gistfile1.rb

# user.rb
class User
  include Mongoid::Document
  devise :database_authenticatable, :registerable,
         :recoverable, :rememberable, :trackable, :validatable
  field :username
  
  .
  .
  .
  
  def to_param
    username
  end
end

# users.rb
FactoryGirl.define do
  factory :user do
    username 'jim_halpert'
    email '[email protected]'
  end
end

# user_spec.rb
require 'spec_helper'

describe User do
  setup do
    @user = Factory(:user)
  end
  
  it "should use the username as the identifier" do
    @user.to_param.should == "jim_halpert"
  end
end

# RESULT
#Failures:
#  1) User should use the username as the identifier
#     Failure/Error: @user.to_param.should == "jim_halpert"
#       expected: "jim_halpert"
#            got: nil (using ==)
#     # ./spec/models/user_spec.rb:9:in `block (2 levels) in <top (required)>'