3 files changed, 29 insertions, 4 deletions
diff --git a/railties/lib/rails/application.rb b/railties/lib/rails/application.rb
index 8075068b3f..e3716992f5 100644
--- a/railties/lib/rails/application.rb
+++ b/railties/lib/rails/application.rb
@@ -514,5 +514,11 @@ module Rails
         end
       end
     end
+
+    private
+
+    def build_middleware
+      config.app_middleware + super
+    end
   end
 end
diff --git a/railties/lib/rails/configuration.rb b/railties/lib/rails/configuration.rb
index d99d27a756..30eafd59f2 100644
--- a/railties/lib/rails/configuration.rb
+++ b/railties/lib/rails/configuration.rb
@@ -33,9 +33,9 @@ module Rails
     #     config.middleware.delete ActionDispatch::Flash
     #
     class MiddlewareStackProxy
-      def initialize
-        @operations = []
-        @delete_operations = []
+      def initialize(operations = [], delete_operations = [])
+        @operations = operations
+        @delete_operations = delete_operations
       end
 
       def insert_before(*args, &block)
@@ -71,6 +71,19 @@ module Rails
 
         other
       end
+
+      def +(other) # :nodoc:
+        MiddlewareStackProxy.new(@operations + other.operations, @delete_operations + other.delete_operations)
+      end
+
+      protected
+        def operations
+          @operations
+        end
+
+        def delete_operations
+          @delete_operations
+        end
     end
 
     class Generators #:nodoc:
diff --git a/railties/lib/rails/engine.rb b/railties/lib/rails/engine.rb
index 5fe96f58ab..457a50e649 100644
--- a/railties/lib/rails/engine.rb
+++ b/railties/lib/rails/engine.rb
@@ -508,7 +508,7 @@ module Rails
     def app
       @app || @app_build_lock.synchronize {
         @app ||= begin
-          config.middleware = config.app_middleware.merge_into(config.middleware).merge_into(default_middleware_stack)
+          config.middleware = build_middleware.merge_into(default_middleware_stack)
           config.middleware.build(endpoint)
         end
       }
@@ -693,5 +693,11 @@ module Rails
     def _all_load_paths #:nodoc:
       @_all_load_paths ||= (config.paths.load_paths + _all_autoload_paths).uniq
     end
+
+    private
+
+    def build_middleware
+      config.middleware
+    end
   end
 end