在mongoose中填充嵌套模型

const mongoose = require('mongoose');
const Schema = mongoose.Schema;

const userSchema = new Schema({
  name: String,
  age: Number
});

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

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

const user = new User({ name: 'John', age: 25 });
user.save();

const post = new Post({ title: 'Hello', content: 'Welcome to my blog!', author: user._id });
post.save();

Post.findOne({ title: 'Hello' })
  .populate('author') // 使用populate方法填充author字段
  .exec((err, post) => {
    if (err) {
      console.error(err);
    } else {
      console.log(post);
      // 输出：
      // {
      //   _id: 123,
      //   title: 'Hello',
      //   content: 'Welcome to my blog!',
      //   author: {
      //     _id: 456,
      //     name: 'John',
      //     age: 25
      //   }
      // }
    }
  });