MongoDB“无法读取null的属性'constructor'”

Question

  const stats = await this.aggregate([
    {
      $match: { tour: tourId },
    },
    {
      $group: {
        _id: '$tour',
        nRating: { $sum: 1 },
        avgRating: { $avg: '$rating' },
},
    },
  ]);
  console.log(stats);

  await Tour.findByIdAndUpdate(tourId, {
    ratingsQuantity: stats[0].nRating,
    ratingsAverage: stats[0].avgRating,
  });
};

reviewSchema.post('save', function () {
  this.constructor.calcAverageRatings(this.tour);
});

reviewSchema.pre(/^findOneAnd/, async function (next) {
  this.r = await this.findOne();
  // console.log(this.r);
  next();
});

reviewSchema.post(/^findOneAnd/, async function () {
  // await this.findOne(); does NOT work here, query has already executed
  await this.r.constructor.calcAverageRatings(this.r.tour);
});

this.r的值为null。因此，分配给this.r的查询返回null，但为reviewSchema的查询中间件预定义了构造函数。