2 files changed, 10 insertions, 5 deletions
diff --git a/actionpack/lib/action_dispatch/routing/mapper.rb b/actionpack/lib/action_dispatch/routing/mapper.rb
index c5e093dae5..c7f6ff620e 100644
--- a/actionpack/lib/action_dispatch/routing/mapper.rb
+++ b/actionpack/lib/action_dispatch/routing/mapper.rb
@@ -28,9 +28,16 @@ module ActionDispatch
             app = app.app
           end
 
+          # Unwrap any constraints so we can see what's inside for route generation.
+          # This allows the formatter to skip over any mounted applications or redirects
+          # that shouldn't be matched when using a url_for without a route name.
+          @dispatcher  = app.is_a?(Routing::RouteSet::Dispatcher)
+
           @app, @constraints, @request = app, constraints, request
         end
 
+        def dispatcher?; @dispatcher; end
+
         def matches?(env)
           req = @request.new(env)
 
diff --git a/actionpack/lib/action_dispatch/routing/route_set.rb b/actionpack/lib/action_dispatch/routing/route_set.rb
index f36453020f..5839db87f9 100644
--- a/actionpack/lib/action_dispatch/routing/route_set.rb
+++ b/actionpack/lib/action_dispatch/routing/route_set.rb
@@ -703,12 +703,10 @@ module ActionDispatch
           end
           old_params = req.path_parameters
           req.path_parameters = old_params.merge params
-          dispatcher = route.app
-          if dispatcher.matches?(env)
-            dispatcher = dispatcher.app
-          end
+          app = route.app
+          if app.matches?(env) && app.dispatcher?
+            dispatcher = app.app
 
-          if dispatcher.is_a?(Dispatcher)
             if dispatcher.controller(params, false)
               dispatcher.prepare_params!(params)
               return params