message: cast unit toString
diff: @@ -28,6 +28,7 @@ const progressSchema = new mongoose.Schema({
  transform: function (doc: any, ret: any) {
  ret._id = ret._id.toString();
  ret.course = ret.course.toString();
  + ret.unit = ret.unit.toString();
  ret.user = ret.user.toString();
  }
  }
  