如何使用mongoose创建对集合的引用

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

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

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

const post = new Post({
  title: 'Sample Post',
  content: 'This is a sample post.',
  author: '60c8e2c6a2d3e80015a3c7f1' // User集合中某个文档的_id
});

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