Topic: mocha current_user

I'm having a problem with writing mocha tests for queries which require the current_user id. 

functional test

def setup
    @admin = login_as(:admin)    # Must login as someone
    @car = Factory.build :car
end

def test_should_show_car
  @car = Car.new(:make => "Nissan", :year => 1998)
  Car.expects(:find).with("123", :user_id => @admin).returns(@car)
  get :show, :id => 123
end

test_helper login_as

def login_as(user)
    @user = Factory.build(user)
    @controller.stubs(:current_user).returns(@user)
    @user
end

Controller

def show
    @car = Car.find(params[:id], :conditions => { :user_id => current_user.id})
end

The problem here is that the car.find command starts looking for a user_id of null.   Should I be going about this another way?