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# \$1size
<a name="size-query"></a>

`$size`查询运算符匹配数组字段具有精确指定数量的元素的文档。这对于根据数组长度筛选文档很有用。

**参数**
+ `field`：要检查的数组字段。
+ `count`：数组必须包含的元素的确切数量。

## 示例（MongoDB 外壳）
<a name="size-query-examples"></a>

以下示例演示如何使用`$size`运算符查找恰好具有三个标签的所有产品。

**创建示例文档**

```
db.products.insertMany([
  { _id: 1, name: "Laptop", tags: ["electronics", "computers", "portable"] },
  { _id: 2, name: "Mouse", tags: ["electronics", "accessories"] },
  { _id: 3, name: "Desk", tags: ["furniture", "office", "workspace"] },
  { _id: 4, name: "Monitor", tags: ["electronics"] }
]);
```

**查询示例**

```
db.products.find({ tags: { $size: 3 } });
```

**输出**

```
{ "_id" : 1, "name" : "Laptop", "tags" : [ "electronics", "computers", "portable" ] }
{ "_id" : 3, "name" : "Desk", "tags" : [ "furniture", "office", "workspace" ] }
```

## 代码示例
<a name="size-query-code"></a>

要查看使用`$size`查询运算符的代码示例，请选择要使用的语言对应的选项卡：

------
#### [ 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('products');

  const result = await collection.find({ tags: { $size: 3 } }).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['products']

    result = list(collection.find({'tags': {'$size': 3}}))

    print(result)
    client.close()

example()
```

------