Re-define _routes when not set correctly via inheritance

actionpack/lib/abstract_controller/railties/routes_helpers.rb

@@ -7,11 +7,27 @@ def self.with(routes, include_path_helpers = true)
         Module.new do
           define_method(:inherited) do |klass|
             super(klass)
-            if namespace = klass.module_parents.detect { |m| m.respond_to?(:railtie_routes_url_helpers) }
+
+            namespace = klass.module_parents.detect { |m| m.respond_to?(:railtie_routes_url_helpers) }
+            actual_routes = namespace ? namespace.railtie_routes_url_helpers._routes : routes
+
+            if namespace
               klass.include(namespace.railtie_routes_url_helpers(include_path_helpers))
             else
               klass.include(routes.url_helpers(include_path_helpers))
             end
+
+            # In the case that we have ex.
+            #   class A::Foo < ApplicationController
+            #   class Bar < A::Foo
+            # We will need to redefine _routes because it will not be correct
+            # via inheritance.
+            unless klass._routes.equal?(actual_routes)
+              klass.redefine_singleton_method(:_routes) { actual_routes }
+              klass.include(Module.new do
+                define_method(:_routes) { @_routes || actual_routes }
+              end)
+            end
           end
         end
       end

railties/test/railties/engine_test.rb

@@ -644,7 +644,7 @@ class Engine < ::Rails::Engine
       assert_equal Rails.application.routes, env["action_dispatch.routes"]
     end
 
-    test "isolated engine should include only its own routes and helpers" do
+    test "isolated engine routes and helpers are isolated to that engine" do
       @plugin.write "lib/bukkits.rb", <<-RUBY
         module Bukkits
           class Engine < ::Rails::Engine
@@ -729,6 +729,30 @@ def polymorphic_path_without_namespace
         end
       RUBY
 
+      @plugin.write "app/controllers/bukkits/session_controller.rb", <<-RUBY
+        module Bukkits
+          class SessionController < ApplicationController
+            def index
+              render plain: default_path
+            end
+
+            private
+              def default_path
+                foo_path
+              end
+          end
+        end
+      RUBY
+
+      controller "bar", <<-RUBY
+        class BarController < Bukkits::SessionController
+          private
+            def default_path
+              bar_path
+            end
+        end
+      RUBY
+
       @plugin.write "app/mailers/bukkits/my_mailer.rb", <<-RUBY
         module Bukkits
           class MyMailer < ActionMailer::Base
@@ -746,6 +770,9 @@ class MyMailer < ActionMailer::Base
       assert ::Bukkits::MyMailer.method_defined?(:foo_url)
       assert_not ::Bukkits::MyMailer.method_defined?(:bar_url)
 
+      get("/bar")
+      assert_equal "/bar", last_response.body
+
       get("/bukkits/from_app")
       assert_equal "false", last_response.body