Mongoose/ Express填充问题

const mongoose = require('mongoose');
const express = require('express');

// 连接MongoDB数据库
mongoose.connect('mongodb://localhost/mydatabase', { useNewUrlParser: true });

// 定义数据模型
const userSchema = new mongoose.Schema({
  name: String,
  age: Number,
  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);

// 创建Express应用程序
const app = express();

// 查询用户数据，并填充相关的帖子数据
app.get('/users', (req, res) => {
  User.find().populate('posts').exec((err, users) => {
    if (err) {
      res.status(500).send(err);
    } else {
      res.send(users);
    }
  });
});

// 启动服务器
app.listen(3000, () => {
  console.log('Server is running on port 3000');
});