如何使用models和MongoDB关联两个模型？

const mongoose = require('mongoose');

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

const postSchema = new mongoose.Schema({
  title: String,
  content: String,
  user: {
    type: mongoose.Schema.Types.ObjectId,
    ref: 'User'
  }
});

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

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

const post = new Post({
  title: 'Hello World',
  content: 'This is my first post',
  user: user._id
});

Post.findOne({ title: 'Hello World' })
  .populate('user')
  .exec((err, post) => {
    if (err) {
      console.error(err);
    } else {
      console.log(post);
    }
  });

const mongoose = require('mongoose');

const bookSchema = new mongoose.Schema({
  title: String,
  price: Number
});

const authorSchema = new mongoose.Schema({
  name: String,
  books: [bookSchema]
});

const Author = mongoose.model('Author', authorSchema);

const author = new Author({
  name: 'John',
  books: [
    { title: 'Book 1', price: 10 },
    { title: 'Book 2', price: 20 }
  ]
});

Author.findOne({ name: 'John' }, (err, author) => {
  if (err) {
    console.error(err);
  } else {
    console.log(author);
  }
});