Update mirror user for all projects deleted user is mirror_user for

app/services/users/destroy_service.rb

@@ -26,6 +26,12 @@ module Users
         ::Projects::DestroyService.new(project, current_user, skip_repo: true).execute
       end
 
+      Project.includes(group: :owners).where(mirror_user: user).find_each do |project|
+        if project.group.present?
+          project.update(mirror_user: project.group.owners.first)
+        end
+      end
+
       MigrateToGhostUserService.new(user).execute unless options[:hard_delete]
 
       # Destroy the namespace after destroying the user since certain methods may depend on the namespace existing

changelogs/unreleased-ee/2532-saving-a-mirrored-project-errors-out-if-mirror-user-has-been-deleted.yml

+---
+title: Update mirror_user for project when mirror_user is deleted
+merge_request: 2013
+author: Athar Hameed

spec/services/users/destroy_service_spec.rb

@@ -159,5 +159,23 @@ describe Users::DestroyService, services: true do
         service.execute(user, hard_delete: true)
       end
     end
+
+    context "when the user was the mirror_user for a group project" do
+      let(:group_owner) { create(:user) }
+      let(:mirror_user) { create(:user) }
+      let(:group)       { create(:group) }
+
+      before do
+        group.add_owner(group_owner)
+        group.add_master(mirror_user)
+      end
+
+      it 'updates the mirror_user to one of the group owners' do
+        project = create(:empty_project, namespace_id: group.id, creator: group_owner, mirror_user: mirror_user)
+        service.execute(mirror_user)
+
+        expect(project.reload.mirror_user).to eq group_owner
+      end
+    end
   end
 end