如何使用dynamic ref填充Mongoos子文档的虚拟数组？

const mongoose = require('mongoose');

const postSchema = new mongoose.Schema({
  title: String,
  content: String
});

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

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

userSchema.virtual('virtuals.posts', {
  ref: function(doc) {
    return 'Post';
  },
  localField: '_id',
  foreignField: 'author'
});

User.findById(userId)
  .populate('virtuals.posts')
  .exec(function(err, user) {
    if (err) {
      console.error(err);
    } else {
      console.log(user);
    }
  });