message: fix courseAdmin id
diff: @@ -65,6 +65,7 @@ const courseSchema = new mongoose.Schema({
  toObject: {
  transform: function (doc: any, ret: any) {
  ret._id = ret._id.toString();
  + ret.courseAdmin = ret.courseAdmin.toString();
  ret.hasAccessKey = false;
  if (ret.accessKey) {
  delete ret.accessKey;
  