使用mongoose的Mongo $lookup嵌套数组3层深度

const mongoose = require('mongoose');

// 连接MongoDB数据库
mongoose.connect('mongodb://localhost/mydatabase', { useNewUrlParser: true, useUnifiedTopology: true });

// 定义相关模型
const UserSchema = new mongoose.Schema({
  username: String,
  // 嵌套数组字段
  posts: [
    {
      title: String,
      comments: [
        {
          content: String,
          likes: Number
        }
      ]
    }
  ]
});

const PostSchema = new mongoose.Schema({
  title: String,
  content: String
});

const CommentSchema = new mongoose.Schema({
  content: String,
  likes: Number
});

const User = mongoose.model('User', UserSchema);
const Post = mongoose.model('Post', PostSchema);
const Comment = mongoose.model('Comment', CommentSchema);

// 使用$lookup进行关联查询
User.aggregate([
  {
    $lookup: {
      from: 'posts',
      localField: 'posts.title',
      foreignField: 'title',
      as: 'joinedPosts'
    }
  },
  {
    $lookup: {
      from: 'comments',
      localField: 'joinedPosts.comments.content',
      foreignField: 'content',
      as: 'joinedComments'
    }
  },
  {
    $lookup: {
      from: 'users',
      localField: 'joinedComments.likes',
      foreignField: 'likes',
      as: 'joinedUsers'
    }
  }
])
.exec((err, result) => {
  if (err) {
    console.log(err);
  } else {
    console.log(result);
  }
});