如何在使用预钩的.save函数之后填充文档

const mongoose = require('mongoose');

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

const PostSchema = new mongoose.Schema({
  title: String,
  content: String
});

const User = mongoose.model('User', UserSchema);
const Post = mongoose.model('Post', PostSchema);

// 在保存文档之前填充文档
UserSchema.pre('save', function(next) {
  this.populate('posts'); // 填充posts字段
  next();
});

// 创建用户
const user = new User({
  name: 'John Doe',
  posts: ['post1', 'post2'] // 这里的post1和post2是Post模型中的文档ID
});

// 保存用户并填充文档
user.save()
  .then(user => {
    console.log(user); // 填充后的用户文档
  })
  .catch(error => {
    console.error(error);
  });