Allow to generate Application routes inside Engine

This requires knowledge about original SCRIPT_NAME and
the parent router. It should be pass through the env
as ORIGIAL_SCRIPT_NAME and action_dispatch.parent_routes

actionpack/lib/action_dispatch/routing/mapper.rb

@@ -289,7 +289,7 @@ def define_generate_prefix(app, name)
             app.routes.class_eval do
               define_method :_generate_prefix do |options|
                 keys = _route.segment_keys + ActionDispatch::Routing::RouteSet::RESERVED_OPTIONS
-                prefix_options = options.reject { |k, v| !(keys).include?(k) }
+                prefix_options = options.slice(*keys)
                 # we must actually delete prefix segment keys to avoid passing them to next url_for
                 _route.segment_keys.each { |k| options.delete(k) }
                 _router.url_helpers.send("#{name}_path", prefix_options)

actionpack/lib/action_dispatch/routing/url_for.rb

@@ -129,9 +129,9 @@ def url_for(options = nil)
           options
         when nil, Hash
           routes = (options ? options.delete(:routes) : nil) || _routes
-
-          if respond_to?(:env) && env && routes.equal?(env["action_dispatch.routes"])
-             options[:skip_prefix] = true
+          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)

actionpack/test/dispatch/prefix_generation_test.rb

@@ -8,6 +8,7 @@ def self.routes
           routes = ActionDispatch::Routing::RouteSet.new
           routes.draw do
             match "/posts/:id", :to => "inside_engine_generating#index", :as => :post
+            match "/bare_url_for", :to => "inside_engine_generating#bare_url_for", :as => :bare_url_for
           end
 
           routes
@@ -37,6 +38,10 @@ def self.routes
 
       def self.call(env)
         env['action_dispatch.routes'] = routes
+
+        # the next to values should be set only in application
+        env['ORIGINAL_SCRIPT_NAME'] = env['SCRIPT_NAME']
+        env['action_dispatch.parent_routes'] = routes
         routes.call(env)
       end
     end
@@ -46,6 +51,14 @@ class ::InsideEngineGeneratingController < ActionController::Base
       def index
         render :text => post_path(:id => params[:id])
       end
+
+      def bare_url_for
+        path = url_for( :routes => RailsApplication.routes,
+                        :controller => "outside_engine_generating", 
+                        :action => "index",
+                        :only_path => true)
+        render :text => path
+      end
     end
 
     class ::OutsideEngineGeneratingController < ActionController::Base
@@ -73,9 +86,8 @@ def foo
     end
 
     test "use SCRIPT_NAME inside the engine" do
-      env = Rack::MockRequest.env_for("/posts/1")
-      env["SCRIPT_NAME"] = "/pure-awesomness/blog"
-      response = ActionDispatch::Response.new(*BlogEngine.call(env))
+      env = Rack::MockRequest.env_for("/pure-awesomness/blog/posts/1")
+      response = ActionDispatch::Response.new(*RailsApplication.call(env))
       assert_equal "/pure-awesomness/blog/posts/1", response.body
     end
 
@@ -98,5 +110,12 @@ def foo
     test "generating urls from a regular class" do
       assert_equal "/awesome/blog/posts/42", Foo.new.foo
     end
+
+    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"
+      response = ActionDispatch::Response.new(*RailsApplication.call(env))
+      assert_equal "/something/generate", response.body
+    end
   end
 end