New way of generating urls for Application from Engine.

It's based specifying application's script_name with:
Rails.application.default_url_options = {:script_name => "/foo"}

default_url_options method is delegated to routes. If router
used to generate url differs from the router passed via env
it always overwrites :script_name with this value.

actionpack/lib/action_controller/metal/url_for.rb

@@ -5,11 +5,20 @@ module ActionController
     include ActionDispatch::Routing::UrlFor
 
     def url_options
-      super.reverse_merge(
+      options = {}
+      if respond_to?(:env) && env
+        if _routes.equal?(env["action_dispatch.routes"])
+          options[:skip_prefix] = true
+        elsif env["action_dispatch.routes"]
+          options[:script_name] = _routes.default_url_options[:script_name]
+        end
+      end
+
+      super.merge(options).reverse_merge(
         :host => request.host_with_port,
         :protocol => request.protocol,
         :_path_segments => request.symbolized_path_parameters
-      ).merge(:script_name => request.script_name)
+      ).reverse_merge(:script_name => request.script_name)
     end
 
     def _routes

actionpack/lib/action_dispatch/routing/route_set.rb

@@ -280,10 +280,10 @@ module ActionDispatch
             # Yes plz - JP
             included do
               routes.install_helpers(self)
-              singleton_class.send(:define_method, :_routes) { routes }
+              singleton_class.send(:define_method, :_routes) { @_routes || routes }
             end
 
-            define_method(:_routes) { routes }
+            define_method(:_routes) { @_routes || routes }
           end
 
           helpers

actionpack/lib/action_dispatch/routing/url_for.rb

@@ -129,16 +129,21 @@ module ActionDispatch
           options
         when nil, Hash
           routes = (options ? options.delete(:routes) : nil) || _routes
-          if respond_to?(:env) && env
-              options[:skip_prefix] = true if routes.equal?(env["action_dispatch.routes"])
-              options[:script_name] = env["ORIGINAL_SCRIPT_NAME"] if routes.equal?(env["action_dispatch.parent_routes"])
-          end
 
-          routes.url_for((options || {}).reverse_merge!(url_options).symbolize_keys)
+          _with_routes(routes) do
+            routes.url_for((options || {}).reverse_merge!(url_options).symbolize_keys)
+          end
         else
           polymorphic_url(options)
         end
       end
+
+      def _with_routes(routes)
+        old_routes, @_routes = @_routes, routes
+        yield
+      ensure
+        @_routes = old_routes
+      end
     end
   end
 end

actionpack/test/controller/routing_test.rb

@@ -251,7 +251,7 @@ class LegacyRouteSetTests < Test::Unit::TestCase
       map.pages 'pages', :controller => 'content', :action => 'show_page', :host => 'foo.com'
     end
     x = setup_for_named_route
-    x.expects(:url_for).with(:host => 'foo.com', :only_path => false, :controller => 'content', :action => 'show_page', :use_route => :pages, :router => rs).once
+    x.expects(:url_for).with(:host => 'foo.com', :only_path => false, :controller => 'content', :action => 'show_page', :use_route => :pages).once
     x.send(:pages_url)
   end
 

actionpack/test/dispatch/prefix_generation_test.rb

@@ -114,6 +114,7 @@ module TestGenerationPrefix
     test "passing :routes to url_for to change current routes" do
       env = Rack::MockRequest.env_for("/pure-awesomness/blog/bare_url_for")
       env["SCRIPT_NAME"] = "/something"
+      RailsApplication.routes.default_url_options = {:script_name => "/something"}
       response = ActionDispatch::Response.new(*RailsApplication.call(env))
       assert_equal "/something/generate", response.body
     end

railties/lib/rails/application.rb

@@ -50,6 +50,8 @@ module Rails
       end
     end
 
+    delegate :default_url_options, :default_url_options=, :to => :routes
+
     # This method is called just after an application inherits from Rails::Application,
     # allowing the developer to load classes in lib and use them during application
     # configuration.
@@ -121,11 +123,6 @@ module Rails
     alias :build_middleware_stack :app
 
     def call(env)
-      if Rails.application == self
-        env["ORIGINAL_SCRIPT_NAME"] = env["SCRIPT_NAME"]
-        env["action_dispatch.parent_routes"] = routes
-      end
-
       env["action_dispatch.routes"] = routes
       app.call(env.reverse_merge!(env_defaults))
     end

railties/test/railties/mounted_engine_routes_test.rb

@@ -99,6 +99,9 @@ module ApplicationTests
       # test generating application's route from engine
       get "/someone/blog/generate_application_route"
       assert_equal "/", last_response.body
+
+      # with script_name
+      Rails.application.default_url_options = {:script_name => "/foo"}
       get "/someone/blog/generate_application_route", {}, "SCRIPT_NAME" => "/foo"
       assert_equal "/foo/", last_response.body
     end