Mongoose findOne无法访问集合的子集合

const mongoose = require('mongoose');

// 定义子集合的Schema
const childSchema = new mongoose.Schema({
  name: String,
  age: Number
});

// 定义顶层集合的Schema
const parentSchema = new mongoose.Schema({
  name: String,
  children: [{ type: mongoose.Schema.Types.ObjectId, ref: 'Child' }]
});

// 定义子集合的Model
const Child = mongoose.model('Child', childSchema);

// 定义顶层集合的Model
const Parent = mongoose.model('Parent', parentSchema);

// 查询顶层集合，并使用populate方法访问子集合
Parent.findOne({ name: 'Parent1' })
  .populate('children')
  .exec((err, parent) => {
    if (err) {
      console.error(err);
      return;
    }
    console.log(parent);
  });