As first step setup the load path and lazy compare middlewares.

actionpack/lib/action_dispatch/middleware/stack.rb

@@ -55,7 +55,11 @@ module ActionDispatch
         when Class
           klass == middleware
         else
-          klass == ActiveSupport::Inflector.constantize(middleware.to_s)
+          if lazy_compare?(@klass) && lazy_compare?(middleware)
+            normalize(@klass) == normalize(middleware)
+          else
+            klass == ActiveSupport::Inflector.constantize(middleware.to_s)
+          end
         end
       end
 
@@ -72,6 +76,14 @@ module ActionDispatch
       end
 
       private
+        def lazy_compare?(object)
+          object.is_a?(String) || object.is_a?(Symbol)
+        end
+
+        def normalize(object)
+          object.to_s.strip.sub(/^::/, '')
+        end
+
         def build_args
           Array(args).map { |arg| arg.respond_to?(:call) ? arg.call : arg }
         end

actionpack/test/dispatch/middleware_stack_test.rb

@@ -87,4 +87,10 @@ class MiddlewareStackTest < ActiveSupport::TestCase
     end
     assert_equal [:foo], @stack.last.send(:build_args)
   end
+
+  test "lazy compares so unloaded constants can be loaded" do
+    @stack.use "UnknownMiddleware"
+    @stack.use :"MiddlewareStackTest::BazMiddleware"
+    assert @stack.include?("::MiddlewareStackTest::BazMiddleware")
+  end
 end

activerecord/lib/active_record/railtie.rb

@@ -49,9 +49,9 @@ module ActiveRecord
     # Setup database middleware after initializers have run
     initializer "active_record.initialize_database_middleware" do |app|
       middleware = app.config.middleware
-      if middleware.include?(ActiveRecord::SessionStore)
-        middleware.insert_before ActiveRecord::SessionStore, ActiveRecord::ConnectionAdapters::ConnectionManagement
-        middleware.insert_before ActiveRecord::SessionStore, ActiveRecord::QueryCache
+      if middleware.include?("ActiveRecord::SessionStore")
+        middleware.insert_before "ActiveRecord::SessionStore", ActiveRecord::ConnectionAdapters::ConnectionManagement
+        middleware.insert_before "ActiveRecord::SessionStore", ActiveRecord::QueryCache
       else
         middleware.use ActiveRecord::ConnectionAdapters::ConnectionManagement
         middleware.use ActiveRecord::QueryCache

railties/lib/rails/application/bootstrap.rb

@@ -76,6 +76,10 @@ module Rails
       initializer :initialize_dependency_mechanism do |app|
         ActiveSupport::Dependencies.mechanism = app.config.cache_classes ? :require : :load
       end
+
+      initializer :bootstrap_load_path do
+        # This is just an initializer used as hook so all load paths are loaded together
+      end
     end
   end
 end

railties/lib/rails/engine.rb

@@ -47,7 +47,7 @@ module Rails
     end
 
     # Add configured load paths to ruby load paths and remove duplicates.
-    initializer :set_load_path do
+    initializer :set_load_path, :before => :bootstrap_load_path do
       config.load_paths.reverse_each do |path|
         $LOAD_PATH.unshift(path) if File.directory?(path)
       end
@@ -56,7 +56,7 @@ module Rails
 
     # Set the paths from which Rails will automatically load source files,
     # and the load_once paths.
-    initializer :set_autoload_paths do |app|
+    initializer :set_autoload_paths, :before => :bootstrap_load_path do |app|
       ActiveSupport::Dependencies.load_paths.unshift(*config.load_paths)
 
       if reloadable?(app)

railties/test/plugins/vendored_test.rb

@@ -48,6 +48,7 @@ module PluginsTest
       RUBY
 
       boot_rails
+      assert $foo
     end
 
     test "plugin paths get added to the AS::Dependency list" do
@@ -252,26 +253,6 @@ YAML
       assert_equal "FooMetal", last_response.body
     end
 
-    test "use plugin middleware in application config" do
-      plugin "foo" do |plugin|
-        plugin.write "lib/foo.rb", <<-RUBY
-          class Foo
-            def initialize(app)
-              @app = app
-            end
-
-            def call(env)
-              @app.call(env)
-            end
-          end
-        RUBY
-      end
-
-      add_to_config "config.middleware.use :Foo"
-
-      boot_rails
-    end
-
     test "namespaced controllers with namespaced routes" do
       @plugin.write "config/routes.rb", <<-RUBY
         ActionController::Routing::Routes.draw do
@@ -332,6 +313,23 @@ YAML
 
       assert rescued, "Expected boot rails to fail"
     end
+
+    test "use plugin middleware in application config" do
+      @plugin.write "lib/bukkits.rb", <<-RUBY
+        class Bukkits
+          def initialize(app)
+            @app = app
+          end
+
+          def call(env)
+            @app.call(env)
+          end
+        end
+      RUBY
+
+      add_to_config "config.middleware.use \"Bukkits\""
+      boot_rails
+    end
   end
 
   class VendoredOrderingTest < Test::Unit::TestCase