Fix mounting engines inside a resources block

When a route is mounted inside a resources block, it's automatically
prefixed, so a following code:

    resources :users do
      mount Blog::Engine => '/blog'
    end

will generate a user_blog path helper.

In order to access engine helpers, we also use "mounted_helpers", a list
of helpers associated with each mounted engine, so a path to blog's post
can be generated using user_blog.post_path(user, post).

The problem I'm fixing here is that mount used a raw :as option, without
taking nestings into account. As a result, blog was added to a route set
as a `user_blog`, but helper was generated for just `blog`.

This commit applies the proper logic for defining a helper for a mounted
engine nested in resources or resource block.

(closes #8533)

1 files changed, 2 insertions, 1 deletions

diff --git a/actionpack/lib/action_dispatch/routing/mapper.rb b/actionpack/lib/action_dispatch/routing/mapper.rb
index 846a6345cb..bfba8d143d 100644
--- a/actionpack/lib/action_dispatch/routing/mapper.rb
+++ b/actionpack/lib/action_dispatch/routing/mapper.rb
@@ -502,11 +502,12 @@ module ActionDispatch
           raise "A rack application must be specified" unless path
 
           options[:as]  ||= app_name(app)
+          target_as       = name_for_action(options[:as], path)
           options[:via] ||= :all
 
           match(path, options.merge(:to => app, :anchor => false, :format => false))
 
-          define_generate_prefix(app, options[:as])
+          define_generate_prefix(app, target_as)
           self
         end