message: Update => UpdateMany
diff: @@ -129,8 +129,8 @@ export class LectureController {
  if (!course.checkPrivileges(currentUser).userCanEditCourse) {
  throw new ForbiddenError();
  }
  - await course.update({}, {$pull: {lectures: id}});
  + await Course.updateMany({}, {$pull: {lectures: id}});
  await Lecture.findByIdAndRemove(id);
  return {};
  