无法填充express和Mongodb中的字段

// 引入依赖
const express = require('express');
const mongoose = require('mongoose');

// 创建Express应用
const app = express();

// 连接MongoDB数据库
mongoose.connect('mongodb://localhost/mydatabase', { useNewUrlParser: true, useUnifiedTopology: true })
  .then(() => {
    console.log('MongoDB连接成功');
  })
  .catch((error) => {
    console.error('MongoDB连接失败', error);
  });

// 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);

// 创建用户和帖子
app.get('/create', async (req, res) => {
  try {
    const post1 = new Post({ title: 'Post 1', content: 'Content 1' });
    const post2 = new Post({ title: 'Post 2', content: 'Content 2' });

    await post1.save();
    await post2.save();

    const user = new User({ name: 'John', age: 25, posts: [post1._id, post2._id] });
    await user.save();

    res.send('用户和帖子创建成功');
  } catch (error) {
    console.error('创建失败', error);
    res.status(500).send('创建失败');
  }
});

// 获取用户和填充帖子字段
app.get('/user/:id', async (req, res) => {
  try {
    const user = await User.findById(req.params.id).populate('posts');
    res.json(user);
  } catch (error) {
    console.error('获取用户失败', error);
    res.status(500).send('获取用户失败');
  }
});

// 启动Express应用
app.listen(3000, () => {
  console.log('Express应用已启动');
});