如何在mongoosejs中获取引用文档中有某些字段的文档

const mongoose = require('mongoose');

const userSchema = new mongoose.Schema({
  name: String,
  email: String
});

const postSchema = new mongoose.Schema({
  title: String,
  content: String,
  author: { type: mongoose.Schema.Types.ObjectId, ref: 'User' }
});

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

Post.aggregate([
  {
    $lookup: {
      from: 'users', // 这里使用的是集合名称，不是模型名称
      localField: 'author',
      foreignField: '_id',
      as: 'authorInfo'
    }
  },
  {
    $match: {
      'authorInfo.email': { $exists: true }
    }
  },
  {
    $project: {
      authorInfo: 0 // 如果你不需要authorInfo字段，可以将其排除
    }
  }
]).exec((err, posts) => {
  if (err) {
    console.error(err);
  } else {
    console.log(posts);
  }
});