MongoDB 查询仅返回嵌入文档

使用MongoDB进行查询时，有时候我们只需要返回嵌入在文档中的特定字段或子文档。这可以通过使用投影操作符来实现。投影操作符可以帮助我们在查询结果中只返回所需的字段，以减少数据传输和提高查询性能。

javascript
{
   "_id": 1,
   "name": "John Doe",
   "age": 30,
   "address": {
      "street": "123 Main St",
      "city": "New York",
      "state": "NY"
   },
   "hobbies": ["reading", "painting", "cooking"]
}

javascript
db.users.find({}, { "name": 1, "address": 1 })

javascript
{
   "_id": 1,
   "name": "John Doe",
   "address": {
      "street": "123 Main St",
      "city": "New York",
      "state": "NY"
   }
}

javascript
db.users.find({}, { "name": 1, "hobbies": { $elemMatch: { $eq: "painting" } } })

javascript
{
   "_id": 1,
   "name": "John Doe",
   "hobbies": ["painting"]
}

javascript
db.users.find({}, { "name": 1, "hobbies": { $slice: 2 } })

javascript
{
   "_id": 1,
   "name": "John Doe",
   "hobbies": ["reading", "painting"]
}

javascript
// 创建一个名为users的集合，并插入示例文档
db.users.insertOne({
   "_id": 1,
   "name": "John Doe",
   "age": 30,
   "address": {
      "street": "123 Main St",
      "city": "New York",
      "state": "NY"
   },
   "hobbies": ["reading", "painting", "cooking"]
});
// 使用投影操作符进行查询，只返回姓名和地址信息
db.users.find({}, { "name": 1, "address": 1 });
// 通过$elemMatch查询嵌套数组，只返回"hobbies"数组中等于"painting"的元素
db.users.find({}, { "name": 1, "hobbies": { $elemMatch: { $eq: "painting" } } });
// 使用$slice限制返回"hobbies"数组的元素数量为2
db.users.find({}, { "name": 1, "hobbies": { $slice: 2 } });