为什么在填充数组时mongoose填充不起作用？

const mongoose = require('mongoose');
mongoose.connect('mongodb://localhost/mydatabase', { useNewUrlParser: true, useUnifiedTopology: true });

const postSchema = new mongoose.Schema({
  title: String,
  content: String,
  author: { type: mongoose.Schema.Types.ObjectId, ref: 'User' }
});

const userSchema = new mongoose.Schema({
  name: String,
  posts: [{ type: mongoose.Schema.Types.ObjectId, ref: 'Post' }]
});

const Post = mongoose.model('Post', postSchema);
const User = mongoose.model('User', userSchema);

// 创建用户和帖子
const newUser = new User({ name: 'John Doe' });
newUser.save((err, user) => {
  if (err) return console.error(err);

  const newPost = new Post({ title: 'My first post', content: 'This is the content', author: user._id });
  newPost.save((err, post) => {
    if (err) return console.error(err);

    // 查询用户并填充帖子
    User.findById(user._id).populate('posts').exec((err, populatedUser) => {
      if (err) return console.error(err);
      console.log(populatedUser); // 包含填充后的帖子
    });
  });
});