如何使用mongoose在mongodb中查询文档

npm install mongoose

const mongoose = require('mongoose');

const connectionString = 'mongodb://localhost:27017/mydatabase';
mongoose.connect(connectionString, { useNewUrlParser: true, useUnifiedTopology: true })
  .then(() => {
    console.log('Connected to MongoDB');
  })
  .catch((error) => {
    console.error('Error connecting to MongoDB', error);
  });

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

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

User.find()
  .then((users) => {
    console.log(users);
  })
  .catch((error) => {
    console.error('Error querying documents', error);
  });

User.find({ age: { $gt: 18 } }) // 查询年龄大于18岁的用户
  .then((users) => {
    console.log(users);
  })
  .catch((error) => {
    console.error('Error querying documents', error);
  });

User.findOne({ name: 'John' }) // 查询名为"John"的用户
  .then((user) => {
    console.log(user);
  })
  .catch((error) => {
    console.error('Error querying document', error);
  });