如何在mongoose中使用$lookup来获取数组对象

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
});

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

User.aggregate([
  {
    $lookup: {
      from: 'posts',
      localField: 'posts',
      foreignField: '_id',
      as: 'userPosts'
    }
  }
])
.exec((err, result) => {
  if (err) {
    console.error(err);
    return;
  }
  console.log(result);
});