如何使用mongoose引用另一个模型中的模型？

const mongoose = require('mongoose');

const userSchema = new mongoose.Schema({
  name: String,
  email: String,
  // 引用另一个模型中的模型
  profile: {
    type: mongoose.Schema.Types.ObjectId,
    ref: 'Profile'
  }
});

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

const profileSchema = new mongoose.Schema({
  bio: String,
  location: String
});

const Profile = mongoose.model('Profile', profileSchema);

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