Allow an absolute controller path inside a module scope

Fixes #12777

3 files changed, 22 insertions, 2 deletions

diff --git a/actionpack/CHANGELOG.md b/actionpack/CHANGELOG.md
index bf608e162b..944bf18113 100644
--- a/actionpack/CHANGELOG.md
+++ b/actionpack/CHANGELOG.md
@@ -1,3 +1,7 @@
+*   Allow an absolute controller path inside a module scope
+
+    Fixes #12777
+
 *   Unique the segment keys array for non-optimized url helpers
 
     In Rails 3.2 you only needed pass an argument for dynamic segment once so
diff --git a/actionpack/lib/action_dispatch/routing/mapper.rb b/actionpack/lib/action_dispatch/routing/mapper.rb
index 4bf2dc6e23..18f37dc732 100644
--- a/actionpack/lib/action_dispatch/routing/mapper.rb
+++ b/actionpack/lib/action_dispatch/routing/mapper.rb
@@ -218,8 +218,12 @@ module ActionDispatch
               controller ||= default_controller
               action     ||= default_action
 
-              unless controller.is_a?(Regexp)
-                controller = [@scope[:module], controller].compact.join("/").presence
+              if @scope[:module] && !controller.is_a?(Regexp)
+                if controller =~ %r{\A/}
+                  controller = controller[1..-1]
+                else
+                  controller = [@scope[:module], controller].compact.join("/").presence
+                end
               end
 
               if controller.is_a?(String) && controller =~ %r{\A/}
diff --git a/actionpack/test/dispatch/routing_test.rb b/actionpack/test/dispatch/routing_test.rb
index 840c157642..5a532dc38f 100644
--- a/actionpack/test/dispatch/routing_test.rb
+++ b/actionpack/test/dispatch/routing_test.rb
@@ -2882,6 +2882,18 @@ class TestRoutingMapper < ActionDispatch::IntegrationTest
     assert_equal '/downloads/1/1.tar', download_path('1', '1')
   end
 
+  def test_absolute_controller_namespace
+    draw do
+      namespace :foo do
+        get '/', to: '/bar#index', as: 'root'
+      end
+    end
+
+    get '/foo'
+    assert_equal 'bar#index', @response.body
+    assert_equal '/foo', foo_root_path
+  end
+
 private
 
   def draw(&block)