如何在MongoDB (mongoose)中没有引用的情况下填充数组字段？

const mongoose = require('mongoose');

const Schema = mongoose.Schema;

const UserSchema = new Schema({
  name: String,
  posts: [{
    type: Schema.Types.ObjectId,
    ref: 'Post'
  }]
});

UserSchema.virtual('populatedPosts', {
  ref: 'Post',
  localField: '_id',
  foreignField: 'author',
  justOne: false,
  options: { sort: { createdAt: -1 } }
});

const User = mongoose.model('User', UserSchema);

module.exports = User;

const user = await User.findById(userId).populate('populatedPosts');

const user = await User.aggregate([
  { $match: { _id: userId } },
  {
    $lookup: {
      from: 'posts',
      localField: '_id',
      foreignField: 'author',
      as: 'populatedPosts'
    }
  }
]);