如何在mongoose上查找基于子填充属性的对象

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.findOne({ name: 'John' })
  .populate('posts')
  .exec((err, user) => {
    if (err) {
      console.error(err);
      return;
    }
    console.log(user.posts);
  });

User.findOne({ name: 'John' })
  .populate('posts', 'title')
  .exec((err, user) => {
    if (err) {
      console.error(err);
      return;
    }
    console.log(user.posts);
  });