Implementing Routing Concerns

This pattern was introduced as a plugin by @dhh.

The original implementation can be found in
https://github.com/rails/routing_concerns

actionpack/lib/action_dispatch/routing/mapper.rb

@@ -909,7 +909,7 @@ module ActionDispatch
         # CANONICAL_ACTIONS holds all actions that does not need a prefix or
         # a path appended since they fit properly in their scope level.
         VALID_ON_OPTIONS  = [:new, :collection, :member]
-        RESOURCE_OPTIONS  = [:as, :controller, :path, :only, :except, :param]
+        RESOURCE_OPTIONS  = [:as, :controller, :path, :only, :except, :param, :concerns]
         CANONICAL_ACTIONS = %w(index create new show update destroy)
 
         class Resource #:nodoc:
@@ -1046,6 +1046,8 @@ module ActionDispatch
           resource_scope(:resource, SingletonResource.new(resources.pop, options)) do
             yield if block_given?
 
+            concerns(options[:concerns]) if options[:concerns]
+
             collection do
               post :create
             end if parent_resource.actions.include?(:create)
@@ -1210,6 +1212,8 @@ module ActionDispatch
           resource_scope(:resources, Resource.new(resources.pop, options)) do
             yield if block_given?
 
+            concerns(options[:concerns]) if options[:concerns]
+
             collection do
               get  :index if parent_resource.actions.include?(:index)
               post :create if parent_resource.actions.include?(:create)
@@ -1580,15 +1584,33 @@ module ActionDispatch
           end
       end
 
+      module Concerns
+        def concern(name, &block)
+          @concerns[name] = block
+        end
+
+        def concerns(*names)
+          names.flatten.each do |name|
+            if concern = @concerns[name]
+              instance_eval(&concern)
+            else
+              raise ArgumentError, "No concern named #{name} was found!"
+            end
+          end
+        end
+      end
+
       def initialize(set) #:nodoc:
         @set = set
         @scope = { :path_names => @set.resources_path_names }
+        @concerns = {}
       end
 
       include Base
       include HttpHelpers
       include Redirection
       include Scoping
+      include Concerns
       include Resources
     end
   end

actionpack/test/dispatch/routing/concerns_test.rb

@@ -0,0 +1,94 @@
+require 'abstract_unit'
+
+class CommentsController < ActionController::Base
+  def index
+    head :ok
+  end
+end
+
+class ImageAttachmentsController < ActionController::Base
+  def index
+    head :ok
+  end
+end
+
+class RoutingConcernsTest < ActionDispatch::IntegrationTest
+  Routes = ActionDispatch::Routing::RouteSet.new.tap do |app|
+    app.draw do
+      concern :commentable do
+        resources :comments
+      end
+
+      concern :image_attachable do
+        resources :image_attachments, only: :index
+      end
+
+      resources :posts, concerns: [:commentable, :image_attachable] do
+        resource :video, concerns: :commentable
+      end
+
+      resource :picture, concerns: :commentable do
+        resources :posts, concerns: :commentable
+      end
+
+      scope "/videos" do
+        concerns :commentable
+      end
+    end
+  end
+
+  include Routes.url_helpers
+  def app; Routes end
+
+  def test_accessing_concern_from_resources
+    get "/posts/1/comments"
+    assert_equal "200", @response.code
+    assert_equal "/posts/1/comments", post_comments_path(post_id: 1)
+  end
+
+  def test_accessing_concern_from_resource
+    get "/picture/comments"
+    assert_equal "200", @response.code
+    assert_equal "/picture/comments", picture_comments_path
+  end
+
+  def test_accessing_concern_from_nested_resource
+    get "/posts/1/video/comments"
+    assert_equal "200", @response.code
+    assert_equal "/posts/1/video/comments", post_video_comments_path(post_id: 1)
+  end
+
+  def test_accessing_concern_from_nested_resources
+    get "/picture/posts/1/comments"
+    assert_equal "200", @response.code
+    assert_equal "/picture/posts/1/comments", picture_post_comments_path(post_id: 1)
+  end
+
+  def test_accessing_concern_from_resources_with_more_than_one_concern
+    get "/posts/1/image_attachments"
+    assert_equal "200", @response.code
+    assert_equal "/posts/1/image_attachments", post_image_attachments_path(post_id: 1)
+  end
+
+  def test_accessing_concern_from_resources_using_only_option
+    get "/posts/1/image_attachment/1"
+    assert_equal "404", @response.code
+  end
+
+  def test_accessing_concern_from_a_scope
+    get "/videos/comments"
+    assert_equal "200", @response.code
+  end
+
+  def test_with_an_invalid_concern_name
+    e = assert_raise ArgumentError do
+      ActionDispatch::Routing::RouteSet.new.tap do |app|
+        app.draw do
+          resources :posts, concerns: :foo
+        end
+      end
+    end
+
+    assert_equal "No concern named foo was found!", e.message
+  end
+end