Testing nested resource controller with rspec in Rails 3.1

I am trying to make a test for a controller for a nested resource.

The nesting is like this in the routes.rb

resources :cars, :only => [:index, :destroy, :show] do
  resources :car_subscriptions, :only => [:new, :create], :as => :follow_subscriptions
end

I'm trying to test the create action most specifically:

describe CarSubscriptionsController do

  def valid_attributes
    {:car_id => '1', :user_id => '2'}
  end

  describe "POST create" do
    describe "with valid params" do
      it "creates a new CarSubscription" do
        expect {
          post :create, :car_id => 1, :car_subscription => valid_attributes
        }.to change(CarSubscription, :count).by(1)
      end

      it "assigns a newly created car_subscription as @car_subscription" do
        post :create, :car_subscription => valid_attributes
        assigns(:car_subscription).should be_a(CarSubscription)
        assigns(:car_subscription).should be_persisted
      end

      it "redirects to the created car_subscription" do
        post :create, :car_subscription => valid_attributes
        response.should redirect_to(CarSubscription.last)
      end
    end
  end

end

It's actually a part of the scaffold generated by rails script. And I only modified the valid_attributes and the post in the first 'it'

And the output is this:

  1) CarSubscriptionsController POST create with valid params creates a new CarSubscription
     Failure/Error: post :create, :car_id => 1, :car_subscription => valid_attributes
     ActionController::RoutingError:
       No route matches {:car_id=>"1", :car_subscription=>{:car_id=>"1", :user_id=>"2"}, :controller=>"car_subscriptions", :action=>"create"}
     # ./spec/controllers/car_subscriptions_controller_spec.rb:34:in `block (5 levels) in <top (required)>'
     # ./spec/controllers/car_subscriptions_controller_spec.rb:33:in `block (4 levels) in <top (required)>'

It's the same error for all 'it's.

I've tried removing the :as => :following_subscriptions from the routes.rb file but the same problem.

I have actually split up the resources of car_subscriptions so index and destroy are in not nested, and create and new are nested in :cars

I don't want to use hard coded paths like in this answer but if it is the only way, I can give it a try:

{ :post => "/forum_topics/1/forum_sub_topics" }.should route_to(:controller => "forum_sub_topics", :action => "create", :forum_topic_id => 1)

EDIT

Oh, and my rake routes looks like this:

car_follow_subscriptions_da POST   /biler/:car_id/car_subscriptions(.:format)                     {:action=>"create", :controller=>"car_subscriptions", :locale=>"da"}