如何填充mongoose集合的嵌套对象？

const userSchema = new mongoose.Schema({
  name: String,
  age: Number,
  address: {
    street: String,
    city: String,
    country: String
  }
});

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

const john = new User({
  name: 'John',
  age: 25,
  address: {
    street: '123 Main St',
    city: 'New York',
    country: 'USA'
  }
});

john.save(function(err) {
  if (err) {
    console.error(err);
  } else {
    console.log('User saved successfully!');
  }
});