diff --git a/app/services/projects/autocomplete_service.rb b/app/services/projects/autocomplete_service.rb
index 3e38a8a12d4..aa60661f7f2 100644
--- a/app/services/projects/autocomplete_service.rb
+++ b/app/services/projects/autocomplete_service.rb
@@ -11,7 +11,7 @@ module Projects
         order: { due_date: :asc, title: :asc }
       }
 
-      finder_params[:group_ids] = @project.group.self_and_ancestors.select(:id) if @project.group
+      finder_params[:group_ids] = @project.group.self_and_ancestors_ids if @project.group
 
       MilestonesFinder.new(finder_params).execute.select([:iid, :title])
     end
diff --git a/changelogs/unreleased/rails5-fix-48012.yml b/changelogs/unreleased/rails5-fix-48012.yml
new file mode 100644
index 00000000000..953ccbd8af7
--- /dev/null
+++ b/changelogs/unreleased/rails5-fix-48012.yml
@@ -0,0 +1,6 @@
+---
+title: Rails5 fix passing Group objects array into for_projects_and_groups milestone
+  scope
+merge_request: 19920
+author: Jasper Maes
+type: fixed