Mongoose：如何查询引用的对象属性

javascript
const mongoose = require('mongoose');
const authorSchema = new mongoose.Schema({
  name: String,
  books: [{ type: mongoose.Schema.Types.ObjectId, ref: 'Book' }]
});
const bookSchema = new mongoose.Schema({
  title: String,
  author: { type: mongoose.Schema.Types.ObjectId, ref: 'Author' }
});
const Author = mongoose.model('Author', authorSchema);
const Book = mongoose.model('Book', bookSchema);

javascript
// 创建作者
const author = new Author({ name: 'John Doe' });
// 创建书籍
const book1 = new Book({ title: 'Book 1', author: author });
const book2 = new Book({ title: 'Book 2', author: author });
// 保存作者和书籍
author.books.push(book1);
author.books.push(book2);
author.save();
book1.save();
book2.save();
// 查询作者并填充书籍
Author.findById(author._id)
  .populate('books')
  .exec(function (err, author) {
    if (err) {
      console.error(err);
    } else {
      console.log(author);
    }
  });