Don't load IdentityMap middleware if not enabled. Simplify middleware.

activerecord/lib/active_record/identity_map.rb

@@ -98,8 +98,9 @@ def initialize(app)
       end
 
       def call(env)
-        ActiveRecord::IdentityMap.enabled? ?
-          ActiveRecord::IdentityMap.use { @app.call(env) } : @app.call(env)
+        ActiveRecord::IdentityMap.use do
+          @app.call(env)
+        end
       end
     end
   end

activerecord/lib/active_record/railtie.rb

@@ -22,9 +22,6 @@ class Railtie < Rails::Railtie
     config.app_middleware.insert_after "::ActionDispatch::Callbacks",
       "ActiveRecord::ConnectionAdapters::ConnectionManagement"
 
-    config.app_middleware.insert_after "::ActionDispatch::Callbacks",
-      "ActiveRecord::IdentityMap::Middleware"
-
     rake_tasks do
       load "active_record/railties/databases.rake"
     end
@@ -81,6 +78,11 @@ class Railtie < Rails::Railtie
       end
     end
 
+    initializer "active_record.identity_map" do |app|
+      config.app_middleware.insert_after "::ActionDispatch::Callbacks",
+        "ActiveRecord::IdentityMap::Middleware" if ActiveRecord::IdentityMap.enabled?
+    end
+
     config.after_initialize do
       ActiveSupport.on_load(:active_record) do
         instantiate_observers

railties/test/application/middleware_test.rb

@@ -107,6 +107,11 @@ def app
       assert_equal "Rack::Runtime", middleware.fourth
     end
 
+    test "identity map is inserted" do
+      boot!
+      assert_equal "ActiveRecord::IdentityMap::Middleware", middleware[9]
+    end
+
     test "insert middleware before" do
       add_to_config "config.middleware.insert_before ActionDispatch::Static, Rack::Config"
       boot!