ruby constant syntax is not supported as routing `:controller` option.

The current implementation only works correctly if you supply the `:controller`
with directory notation (eg. `:controller => 'admin/posts'`).

The ruby constant notation (eg. `:controller => 'Admin::Posts`) leads to unexpected problems with `url_for`.

This patch prints a warning for every non supported `:controller` option. I also added documentation how
to work with namespaced controllers. The warning links to that documentation in the rails guide.

1 files changed, 6 insertions, 0 deletions

diff --git a/actionpack/lib/action_dispatch/routing/mapper.rb b/actionpack/lib/action_dispatch/routing/mapper.rb
index 82ef1d0333..34f5f80d4d 100644
--- a/actionpack/lib/action_dispatch/routing/mapper.rb
+++ b/actionpack/lib/action_dispatch/routing/mapper.rb
@@ -246,6 +246,12 @@ module ActionDispatch
                 raise ArgumentError, "missing :action"
               end
 
+              if controller.is_a?(String) && controller !~ /\A[a-z_\/]+\z/
+                message = "'#{controller}' is not a supported controller name. This can lead to potential routing problems."
+                message << " See http://guides.rubyonrails.org/routing.html#specifying-a-controller-to-use"
+                raise ArgumentError, message
+              end
+
               hash = {}
               hash[:controller] = controller unless controller.blank?
               hash[:action]     = action unless action.blank?