Delete the notification when the WP gets destroyed

app/services/work_packages/delete_service.rb

@@ -44,6 +44,7 @@ class WorkPackages::DeleteService < ::BaseServices::Delete
       end
 
       destroy_descendants(descendants, result)
+      delete_associated(model)
     end
 
     result
@@ -58,4 +59,14 @@ class WorkPackages::DeleteService < ::BaseServices::Delete
       result.add_dependent!(ServiceResult.new(success: descendant.destroy, result: descendant))
     end
   end
+
+  def delete_associated(model)
+    delete_notifications_resource(model.id)
+  end
+
+  def delete_notifications_resource(id)
+    Notification
+      .where(resource_type: :WorkPackage, resource_id: id)
+      .delete_all
+  end
 end

spec/services/work_packages/delete_service_integration_spec.rb

@@ -92,4 +92,28 @@ describe WorkPackages::DeleteService, 'integration', type: :model do
       expect { work_package.reload }.to raise_error(ActiveRecord::RecordNotFound)
     end
   end
+
+  describe 'with a notification' do
+    let!(:work_package) { FactoryBot.create :work_package, project: project }
+    let!(:notification) do
+      FactoryBot.create :notification,
+                        recipient: user,
+                        actor: user,
+                        resource: work_package,
+                        project: project
+    end
+
+    let(:instance) do
+      described_class.new(user: user,
+                          model: work_package)
+    end
+
+    subject { instance.call }
+
+    it 'deletes the notification' do
+      expect(subject).to be_success
+      expect { work_package.reload }.to raise_error(ActiveRecord::RecordNotFound)
+      expect { notification.reload }.to raise_error(ActiveRecord::RecordNotFound)
+    end
+  end
 end