理解Mongoose中的关系和外键

const mongoose = require('mongoose');
mongoose.connect('mongodb://localhost:27017/blog', { useNewUrlParser: true, useUnifiedTopology: true });

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

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

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

// 创建一个用户
const user = new User({ name: 'John Doe', email: 'john@example.com' });
user.save((err, savedUser) => {
  if (err) return console.error(err);

  // 创建一篇文章并引用该用户
  const post = new Post({ title: 'My First Post', content: 'This is the content of my first post.', author: savedUser._id });
  post.save((err, savedPost) => {
    if (err) return console.error(err);
    console.log('Post saved:', savedPost);
  });
});

Post.findById(postId).populate('author').exec((err, post) => {
  if (err) return console.error(err);
  console.log('Post with author:', post);
});

const commentSchema = new mongoose.Schema({
  content: String,
  post: { type: mongoose.Schema.Types.ObjectId, refPath: 'postModel' }
});

const postSchema = new mongoose.Schema({
  title: String,
  content: String,
  comments: [{ type: commentSchema }]
});

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