无法保存引用了另一个对象的mongoose模型

const mongoose = require('mongoose');

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

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

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

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

// 创建一个新的User对象
const user = new User({
  name: 'John',
  age: 25
});

// 保存User对象到数据库
user.save()
  .then(() => {
    // 创建一个新的Post对象，并将user对象作为author引用
    const post = new Post({
      title: 'Hello World',
      content: 'This is my first post',
      author: user._id
    });

    // 保存Post对象到数据库
    post.save()
      .then(() => {
        console.log('Post saved successfully');
      })
      .catch((error) => {
        console.error('Error saving post:', error);
      });
  })
  .catch((error) => {
    console.error('Error saving user:', error);
  });