Extract EE-specific code from app/controllers/groups/milestones_controller.rb

app/controllers/groups/milestones_controller.rb

 class Groups::MilestonesController < Groups::ApplicationController
+  prepend EE::Groups::MilestonesController
+
   include MilestoneActions
 
   before_action :group_projects
@@ -77,17 +79,14 @@ class Groups::MilestonesController < Groups::ApplicationController
   def milestones
     milestones = MilestonesFinder.new(search_params).execute
 
-    legacy_milestones =
-      if params[:only_group_milestones]
-        []
-      else
-        GroupMilestone.build_collection(group, group_projects, params)
-      end
-
     @sort = params[:sort] || 'due_date_asc'
     MilestoneArray.sort(milestones + legacy_milestones, @sort)
   end
 
+  def legacy_milestones
+    GroupMilestone.build_collection(group, group_projects, params)
+  end
+
   def milestone
     @milestone =
       if params[:title]

ee/app/controllers/ee/groups/milestones_controller.rb

+module EE
+  module Groups
+    module MilestonesController
+      extend ::Gitlab::Utils::Override
+
+      override :legacy_milestones
+      def legacy_milestones
+        params[:only_group_milestones] ? [] : super
+      end
+    end
+  end
+end