如何在express中引用其他集合中的文档

npm install mongoose

const mongoose = require('mongoose');

mongoose.connect('mongodb://localhost/mydatabase', { useNewUrlParser: true, useUnifiedTopology: true });

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

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

// 查询所有用户
User.find({}, (err, users) => {
  if (err) {
    console.error(err);
  } else {
    console.log(users);
  }
});

// 查询年龄大于等于18岁的用户
User.find({ age: { $gte: 18 } }, (err, users) => {
  if (err) {
    console.error(err);
  } else {
    console.log(users);
  }
});

// 创建一个新用户
const newUser = new User({ name: 'John', age: 25 });
newUser.save((err) => {
  if (err) {
    console.error(err);
  } else {
    console.log('User created successfully');
  }
});