如何从mongodb中查询和获取数组中的一项？

db.students.find({ "_id": student_id }, { "grades.0": 1, "_id": 0 })

const MongoClient = require('mongodb').MongoClient;
const uri = "mongodb+srv://<username>:<password>@cluster0.mongodb.net/test?retryWrites=true&w=majority";
const client = new MongoClient(uri, { useNewUrlParser: true, useUnifiedTopology: true });

client.connect(err => {
  const collection = client.db("test").collection("students");
  const query = { "_id": student_id };
  const projection = { "grades.0": 1, "_id": 0 };

  collection.findOne(query, { projection }, (err, result) => {
    if (err) throw err;
    console.log(result.grades[0]); // 打印第一个成绩
    client.close();
  });
});