查询子文档字典MongoDB中的值

{
  "_id": ObjectId("615f4d6b3d0e5a001f9e5e8a"),
  "name": "John Doe",
  "age": 30,
  "address": {
    "street": "123 Main St",
    "city": "New York",
    "state": "NY",
    "country": "USA"
  }
}

// 连接到MongoDB数据库
const MongoClient = require('mongodb').MongoClient;
const uri = 'mongodb://localhost:27017';
const client = new MongoClient(uri, { useNewUrlParser: true, useUnifiedTopology: true });

// 查询子文档字典中的值
client.connect(err => {
  if (err) throw err;
  
  const collection = client.db('mydb').collection('mycollection');
  
  // 查询name字段的值
  collection.findOne({}, { projection: { name: 1 } }, (err, result) => {
    if (err) throw err;
    console.log(result.name);
  });
  
  // 查询address子文档中的city字段的值
  collection.findOne({}, { projection: { 'address.city': 1 } }, (err, result) => {
    if (err) throw err;
    console.log(result.address.city);
  });
  
  client.close();
});