How do I write a Rails 3.1 engine controller test in rspec?

I have written a Rails 3.1 engine with the namespace Posts. Hence, my controllers are found in app/controllers/posts/, my models in app/models/posts, etc. I can test the models just fine. The spec for one model looks like...

module Posts
  describe Post do
    describe 'Associations' do
      it ...
      end

... and everything works fine.

However, the specs for the controllers do not work. The Rails engine is mounted at /posts, yet the controller is Posts::PostController. Thus, the tests look for the controller route to be posts/posts.

  describe "GET index" do
    it "assigns all posts as @posts" do
      Posts::Post.stub(:all) { [mock_post] }
       get :index
       assigns(:posts).should eq([mock_post])
    end
  end

which yields...

  1) Posts::PostsController GET index assigns all posts as @posts
     Failure/Error: get :index
     ActionController::RoutingError:
     No route matches {:controller=>"posts/posts"}
     # ./spec/controllers/posts/posts_controller_spec.rb:16

I've tried all sorts of tricks in the test app's routes file... :namespace, etc, to no avail.

How do I make this work? It seems like it won't, since the engine puts the controller at /posts, yet the namespacing puts the controller at /posts/posts for the purpose of testing.