FactoryGirl: attributes_for not giving me associated attributes

I have a Code model factory like this:

Factory.define :code do |f|
    f.value "code"
    f.association :code_type
    f.association(:codeable, :factory => :portfolio)
end

But when I test my controller with a simple test_should_create_code like this:

  test "should create code" do
    assert_difference('Code.count') do
      post :create, :code => Factory.attributes_for(:code)
    end
    assert_redirected_to code_path(assigns(:code))
  end

... the test fails. The new record is not created.

In the console, it seems that attributes_for does not return all required attributes like the create does.

rob@compy:~/dev/my_rails_app$ rails console test
Loading test environment (Rails 3.0.3)
irb(main):001:0> Factory.create(:code)
=> #<Code id: 1, code_type_id: 1, value: "code", codeable_id: 1, codeable_type: "Portfolio", created_at: "2011-02-24 10:42:20", updated_at: "2011-02-24 10:42:20">
irb(main):002:0> Factory.attributes_for(:code)
=> {:value=>"code"}

Any ideas?

Thanks,