如何将字符串与同一Mongoose Schema中的另一个键值连接起来

const mongoose = require('mongoose');

const schema = new mongoose.Schema({
  firstName: String,
  lastName: String
});

// 定义虚拟属性 fullName
schema.virtual('fullName').get(function() {
  return this.firstName + ' ' + this.lastName;
});

const Model = mongoose.model('Model', schema);

// 使用虚拟属性
Model.findOne({}, (err, doc) => {
  console.log(doc.fullName); // 输出连接后的字符串
});