如何在nodejs mongodb中使用引用对象id

const mongoose = require('mongoose');
mongoose.connect('mongodb://localhost/mydatabase', { useNewUrlParser: true, useUnifiedTopology: true })
  .then(() => {
    console.log('Connected to MongoDB');
  })
  .catch((error) => {
    console.error('Error connecting to MongoDB', error);
  });

const mongoose = require('mongoose');

const userSchema = new mongoose.Schema({
  name: String,
  age: Number,
  address: {
    type: mongoose.Schema.Types.ObjectId,
    ref: 'Address'
  }
});

const addressSchema = new mongoose.Schema({
  city: String,
  country: String
});

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

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

const address = new Address({
  city: 'New York',
  country: 'USA'
});

address.save((error, savedAddress) => {
  if (error) {
    console.error('Error saving address', error);
  } else {
    const user = new User({
      name: 'John',
      age: 30,
      address: savedAddress._id
    });

    user.save((error, savedUser) => {
      if (error) {
        console.error('Error saving user', error);
      } else {
        console.log('User saved:', savedUser);
      }
    });
  }
});