Fix generators to help with ambiguous `ApplicationController` issue

In development mode, dependencies are loaded dynamically at runtime,
using `const_missing`. Because of that, when one of the constants is
already loaded and `const_missing` is not triggered, user can end up
with unexpected results.

Given such file in an Engine:

```ruby
module Blog
  class PostsController < ApplicationController
  end
end
```

If you load it first, before loading any application files, it will
correctly load `Blog::ApplicationController`, because second line will
hit `const_missing`. However if you load `ApplicationController` first,
the constant will be loaded already, `const_missing` hook will not be
fired and in result `PostsController` will inherit from
`ApplicationController` instead of `Blog::ApplicationController`.

Since it can't be fixed in `AS::Dependencies`, the easiest fix is to
just explicitly load application controller.

closes #6413

3 files changed, 12 insertions, 0 deletions

diff --git a/railties/lib/rails/generators/named_base.rb b/railties/lib/rails/generators/named_base.rb
index 99735c49c4..34b316b964 100644
--- a/railties/lib/rails/generators/named_base.rb
+++ b/railties/lib/rails/generators/named_base.rb
@@ -82,6 +82,10 @@ module Rails
           @class_path
         end
 
+        def namespaced_file_path
+          @namespaced_file_path ||= namespaced_class_path.join("/")
+        end
+
         def namespaced_class_path
           @namespaced_class_path ||= begin
             namespace_path = namespace.name.split("::").map {|m| m.underscore }
diff --git a/railties/lib/rails/generators/rails/controller/templates/controller.rb b/railties/lib/rails/generators/rails/controller/templates/controller.rb
index 52243f4a2f..9b04192126 100644
--- a/railties/lib/rails/generators/rails/controller/templates/controller.rb
+++ b/railties/lib/rails/generators/rails/controller/templates/controller.rb
@@ -1,3 +1,7 @@
+<% if namespaced? -%>
+require "<%= namespaced_file_path %>/application_controller"
+<% end -%>
+
 <% module_namespacing do -%>
 class <%= class_name %>Controller < ApplicationController
 <% actions.each do |action| -%>
diff --git a/railties/lib/rails/generators/rails/scaffold_controller/templates/controller.rb b/railties/lib/rails/generators/rails/scaffold_controller/templates/controller.rb
index 4ff15fd288..dc0f5a0b3b 100644
--- a/railties/lib/rails/generators/rails/scaffold_controller/templates/controller.rb
+++ b/railties/lib/rails/generators/rails/scaffold_controller/templates/controller.rb
@@ -1,3 +1,7 @@
+<% if namespaced? -%>
+require "<%= namespaced_file_path %>/application_controller"
+<% end -%>
+
 <% module_namespacing do -%>
 class <%= controller_class_name %>Controller < ApplicationController
   # GET <%= route_url %>