Mongoose，按填充字段对查询进行排序

使用Mongoose对查询结果进行排序是一种非常有用的技巧，它可以让我们按照指定的填充字段对结果进行排序。在本文中，我们将探讨如何使用Mongoose来实现这一功能，并提供一些案例代码来帮助读者更好地理解。

javascript
const mongoose = require('mongoose');
const postSchema = new mongoose.Schema({
  title: String,
  content: String
});
const userSchema = new mongoose.Schema({
  name: String,
  posts: [{ type: mongoose.Schema.Types.ObjectId, ref: 'Post' }]
});
const User = mongoose.model('User', userSchema);
const Post = mongoose.model('Post', postSchema);

javascript
User.find()
  .populate('posts')
  .sort({'posts.title': 1})
  .exec((err, users) => {
    if (err) {
      console.error(err);
    } else {
      console.log(users);
    }
  });

javascript
const mongoose = require('mongoose');
const userSchema = new mongoose.Schema({
  name: String,
  posts: [{ type: mongoose.Schema.Types.ObjectId, ref: 'Post' }]
});
const postSchema = new mongoose.Schema({
  title: String,
  content: String,
  user: { type: mongoose.Schema.Types.ObjectId, ref: 'User' }
});
const User = mongoose.model('User', userSchema);
const Post = mongoose.model('Post', postSchema);

javascript
User.aggregate([
  { $lookup: { from: 'posts', localField: 'posts', foreignField: '_id', as: 'posts' } },
  { $addFields: { numPosts: { $size: '$posts' } } },
  { $sort: { numPosts: -1 } }
]).exec((err, users) => {
  if (err) {
    console.error(err);
  } else {
    console.log(users);
  }
});