如何使用mongoose创建和更新引用的文档？

const mongoose = require('mongoose');

const userSchema = new mongoose.Schema({
  name: String,
  // 其他字段...
});

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

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

const user = new User({
  name: 'John Doe'
  // 其他字段...
});

user.save((err, savedUser) => {
  if (err) {
    console.error(err);
  } else {
    // 创建一个Post实例，并将user实例作为引用赋值给author字段
    const post = new Post({
      title: 'Hello World',
      content: 'This is a sample post',
      author: savedUser._id
      // 其他字段...
    });

    post.save((err, savedPost) => {
      if (err) {
        console.error(err);
      } else {
        console.log('Post saved:', savedPost);
      }
    });
  }
});

Post.findByIdAndUpdate(postId, { author: newAuthorId }, (err, updatedPost) => {
  if (err) {
    console.error(err);
  } else {
    console.log('Post updated:', updatedPost);
  }
});