如何仅在mongoose中使用聚合填充嵌套在对象数组中的字段？

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', // 与之关联的集合名
      localField: 'posts',
      foreignField: '_id',
      as: 'populatedPosts'
    }
  }
])
.exec((err, users) => {
  if (err) {
    console.error(err);
  } else {
    console.log(users);
  }
});

User.aggregate([
  {
    $lookup: {
      from: 'posts',
      localField: 'posts',
      foreignField: '_id',
      as: 'populatedPosts'
    }
  },
  {
    $project: {
      name: 1,
      populatedPosts: 1
    }
  }
])
.exec((err, users) => {
  if (err) {
    console.error(err);
  } else {
    console.log(users);
  }
});