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# \$1slice
<a name="slice-projection"></a>

`$slice`投影运算符限制查询结果中返回的数组元素的数量。它允许您从数组字段的开头或结尾检索特定数量的元素，而无需加载整个数组。

**参数**
+ `field`：要投影的数组字段。
+ `count`：要返回的元素数量。正值从一开始返回元素，从结尾返回负值。

## 示例（MongoDB 外壳）
<a name="slice-projection-examples"></a>

以下示例演示如何使用`$slice`投影运算符仅返回数组字段中的前两项。

**创建示例文档**

```
db.inventory.insertMany([
  { _id: 1, item: "notebook", tags: ["office", "school", "supplies", "writing"] },
  { _id: 2, item: "pen", tags: ["office", "writing"] },
  { _id: 3, item: "folder", tags: ["office", "supplies", "storage", "organization"] }
]);
```

**查询示例**

```
db.inventory.find(
  {},
  { item: 1, tags: { $slice: 2 } }
)
```

**输出**

```
{ "_id" : 1, "item" : "notebook", "tags" : [ "office", "school" ] }
{ "_id" : 2, "item" : "pen", "tags" : [ "office", "writing" ] }
{ "_id" : 3, "item" : "folder", "tags" : [ "office", "supplies" ] }
```

## 代码示例
<a name="slice-projection-code"></a>

要查看使用`$slice`投影运算符的代码示例，请选择要使用的语言的选项卡：

------
#### [ Node.js ]

```
const { MongoClient } = require('mongodb');

async function example() {
  const client = await MongoClient.connect('mongodb://<username>:<password>@<cluster-endpoint>:27017/?tls=true&tlsCAFile=global-bundle.pem&replicaSet=rs0&readPreference=secondaryPreferred&retryWrites=false');
  const db = client.db('test');
  const collection = db.collection('inventory');

  const result = await collection.find(
    {},
    { projection: { item: 1, tags: { $slice: 2 } } }
  ).toArray();

  console.log(JSON.stringify(result, null, 2));
  await client.close();
}

example();
```

------
#### [ Python ]

```
from pymongo import MongoClient

def example():
    client = MongoClient('mongodb://<username>:<password>@<cluster-endpoint>:27017/?tls=true&tlsCAFile=global-bundle.pem&replicaSet=rs0&readPreference=secondaryPreferred&retryWrites=false')
    db = client['test']
    collection = db['inventory']

    result = list(collection.find(
        {},
        {'item': 1, 'tags': {'$slice': 2}}
    ))

    print(result)
    client.close()

example()
```

------