ruby-on-rails - Rspec：实例类上的 should_receive

Question

这是我正在尝试做的事情：

before(:each) do
    @user = Factory.build(:user)
    @user.stub!(:must_not_be_in_somewhere).and_return(true)
 end

it "should send a email if we are 2 weeks from last email" do
    User.any_instance.should_receive(:deliver_promo_code).exactly(111).times

    post :create, :user => @user.attributes, :format => :json
 end

代码有效，但测试并没有失败……任何人都知道该代码有什么问题吗？