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

// 定义虚拟字段
userSchema.virtual('fullName').get(function() {
  return `${this.firstName} ${this.lastName}`;
});

userSchema.set('toJSON', {
  virtuals: true
});

// 或者
userSchema.set('toObject', {
  virtuals: true
});

const user = await User.findById(id).lean({ virtuals: true });

const user = await User.findById(id);
const userObj = user.toObject({ virtuals: true });
res.json(userObj);

const mongoose = require('mongoose');
const express = require('express');
const app = express();

// 连接数据库
mongoose.connect('mongodb://localhost:27017/test');

// 定义Schema和模型
const userSchema = new mongoose.Schema({
  firstName: String,
  lastName: String,
  createdAt: { type: Date, default: Date.now }
});

// 定义虚拟字段
userSchema.virtual('fullName').get(function() {
  return `${this.firstName} ${this.lastName}`;
});

// 定义另一个虚拟字段，计算用户年龄
userSchema.virtual('age').get(function() {
  const diff = Date.now() - this.createdAt.getTime();
  return Math.floor(diff / (1000 * 60 * 60 * 24 * 365)); // 粗略计算年数
});

// 启用虚拟字段在JSON输出中
userSchema.set('toJSON', {
  virtuals: true,
  transform: (doc, ret) => {
    delete ret._id;
    delete ret.__v;
    return ret;
  }
});

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

// API路由
app.get('/users/:id', async (req, res) => {
  try {
    const user = await User.findById(req.params.id);
    if (!user) {
      return res.status(404).json({ message: 'User not found' });
    }
    res.json(user);
  } catch (err) {
    res.status(500).json({ message: err.message });
  }
});

// 启动服务器
app.listen(3000, () => {
  console.log('Server running on port 3000');
});