Route works in browser, fails in Rspec - ruby-on-rails

Question

This question has probably been asked a dozen times on Stack Overflow (e.g. (1), (2), (3), (4), (5)) but every time the answer seems to be different and none of them have helped me. I'm working on a Rails Engine and I'm finding that Rspec2 gets route errors, but I can reach the routes in the browser. Here's the situation:
In the engine's routes.rb:
resources :mw_interactives, :controller => 'mw_interactives', :constraints => { :id => /\d+/ }, :except => :show
# This is so we can build the InteractiveItem at the same time as the Interactive
resources :pages, :controller => 'interactive_pages', :constraints => { :id => /\d+/ }, :only => [:show] do
resources :mw_interactives, :controller => 'mw_interactives', :constraints => { :id => /\d+/ }, :except => :show
end
Excerpted output of rake routes:
new_mw_interactive GET /mw_interactives/new(.:format) lightweight/mw_interactives#new {:id=>/\d+/}
...
new_page_mw_interactive GET /pages/:page_id/mw_interactives/new(.:format) lightweight/mw_interactives#new {:id=>/\d+/, :page_id=>/\d+/}
And my test, from one of the controller specs (describe Lightweight::MwInteractivesController do):
it 'shows a form for a new interactive' do
get :new
end
...which gets this result:
Failure/Error: get :new
ActionController::RoutingError:
No route matches {:controller=>"lightweight/mw_interactives", :action=>"new"}
...and yet when I go to that route in the browser, it works exactly as intended.
What am I missing here?
ETA: To clarify a point Andreas raises: this is a Rails Engine, so rspec runs in a dummy application which includes the engine's routes in a namespace:
mount Lightweight::Engine => "/lightweight"
...so the routes shown in rake routes are prefaced with /lightweight/. That's why the route shown in the Rspec error doesn't seem to match what's in rake routes. But it does make the debugging an extra step wonkier.
ETA2: Answering Ryan Clark's comment, this is the action I'm testing:
module Lightweight
class MwInteractivesController < ApplicationController
def new
create
end
...and that's it.

Answer 1

I found a workaround for this. Right at the top of the spec, I added this code:
render_views
before do
# work around bug in routing testing
#routes = Lightweight::Engine.routes
end
...and now the spec runs without the routing error. But I don't know why this works, so if someone can post an answer which explains it, I'll accept that.

Answer 2

I think the might be something wrong higher up in you specs
how did the "lightweight" get into this line :controller=>"lightweight/mw_interactives"
the route says
new_mw_interactive GET /mw_interactives/new(.:format)
not
new_mw_interactive GET /lightweight/mw_interactives/new(.:format)

Answer 3

add a file spec/routing/root_routing_spec.rb
require "spec_helper"
describe "routes for Widgets" do
it "routes /widgets to the widgets controller" do
{ :get => "/" }.should route_to(:controller => "home", :action => "index")
end
end
then add a file spec/controllers/home_controller_spec.rb
require 'spec_helper'
describe HomeController do
context "GET index" do
before(:each) do
get :index
end
it {should respond_with :success }
it {should render_template(:index) }
end
end