如何在node和mongoose中使用嵌套填充

const mongoose = require('mongoose');

// 创建Author模型
const authorSchema = new mongoose.Schema({
  name: String,
  age: Number
});

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

// 创建Book模型，并嵌套填充Author模型
const bookSchema = new mongoose.Schema({
  title: String,
  author: {
    type: mongoose.Schema.Types.ObjectId,
    ref: 'Author'
  }
});

const Book = mongoose.model('Book', bookSchema);

// 创建一个作者
const author = new Author({
  name: 'John Doe',
  age: 30
});

// 保存作者到数据库
author.save((err, savedAuthor) => {
  if (err) {
    console.error(err);
    return;
  }

  // 创建一本书，并嵌套填充作者ID
  const book = new Book({
    title: 'Sample Book',
    author: savedAuthor._id
  });

  // 保存书籍到数据库
  book.save((err, savedBook) => {
    if (err) {
      console.error(err);
      return;
    }

    console.log('Book saved:', savedBook);
  });
});

// 查询书籍，并填充作者信息
Book.findOne({ title: 'Sample Book' })
  .populate('author')
  .exec((err, book) => {
    if (err) {
      console.error(err);
      return;
    }

    console.log('Book with author:', book);
  });