Testing nested resources with RSpec

I am trying to create tests for nested resources in Rails. The relevant route definition is:

resources :communities do
  resources :contents, :type => 'Content'
end

Using RSpec and factory_girl, I am trying to get started with testing with e.g.

describe ContentsController do
  it 'should display a content item under a community' do
    content = FactoryGirl.create(:content)
    get :show, :community_id => content.community.id, :id => content.id
  end
end

These requests always result in

Failure/Error: get :show, :community_id => content.community.id, :id => content.id
 ActionController::RoutingError:
   No route matches {:community_id=>BSON::ObjectId('4e7773c6ac54c3d1ad000002'),
   :id=>BSON::ObjectId('4e7773c6ac54c3d1ad000001'), :controller=>"contents",
   :action=>"show"}

For the life of me I cannot find a way to specify a route to a nested resource with RSpec. Am I doing something fundamentally wrong here?

Update: The relevant part of rake routes is:

    community_contents GET    /communities/:community_id/contents(.:format)             {:action=>"index", :controller=>"contents"}
                       POST   /communities/:community_id/contents(.:format)             {:action=>"create", :controller=>"cont开发者_JAVA技巧ents"}
 new_community_content GET    /communities/:community_id/contents/new(.:format)         {:action=>"new", :controller=>"contents"}
edit_community_content GET    /communities/:community_id/contents/:id/edit(.:format)    {:action=>"edit", :controller=>"contents"}
     community_content GET    /communities/:community_id/contents/:id(.:format)         {:action=>"show", :controller=>"contents"}
                       PUT    /communities/:community_id/contents/:id(.:format)         {:action=>"update", :controller=>"contents"}
                       DELETE /communities/:community_id/contents/:id(.:format)         {:action=>"destroy", :controller=>"contents"}

I see that you are passing the content.community.id as the :community_id and that object looks like a mongo document that is identified with a BSON::ObjectId. Try to use to_param instead as following:

get :show, :community_id => content.community.to_param, :id => content.to_param