Mongoose填充多个数组中的嵌套元素

const mongoose = require('mongoose');

// 定义模式
const userSchema = new mongoose.Schema({
  name: String,
  posts: [{
    type: mongoose.Schema.Types.ObjectId,
    ref: 'Post'
  }]
});

const postSchema = new mongoose.Schema({
  title: String,
  comments: [{
    type: mongoose.Schema.Types.ObjectId,
    ref: 'Comment'
  }]
});

const commentSchema = new mongoose.Schema({
  content: String
});

// 定义模型
const User = mongoose.model('User', userSchema);
const Post = mongoose.model('Post', postSchema);
const Comment = mongoose.model('Comment', commentSchema);

// 填充嵌套元素
User.findOne({ name: 'John' })
  .populate({
    path: 'posts',
    populate: {
      path: 'comments'
    }
  })
  .exec((err, user) => {
    if (err) {
      console.error(err);
    } else {
      console.log(user);
    }
  });