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# \$1toLower
<a name="toLower"></a>

Amazon DocumentDB 中的`$toLower`运算符用于将字符串转换为小写。

**参数**
+ `expression`: 要转换为小写的字符串表达式。

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

以下示例演示如何使用`$toLower`运算符将`Desk`字段转换为小写。

**创建示例文档**

```
db.locations.insertMany([
  { "_id": 1, "Desk": "Düsseldorf-BVV-021" },
  { "_id": 2, "Desk": "Munich-HGG-32a" }
]);
```

**查询示例**

```
db.locations.aggregate([
  { $project: { item: { $toLower: "$Desk" } } }
]);
```

**输出**

```
{ "_id" : 1, "item" : "düsseldorf-bvv-021" }
{ "_id" : 2, "item" : "munich-hgg-32a" }
```

## 代码示例
<a name="toLower-code"></a>

要查看使用该`$toLower`命令的代码示例，请选择要使用的语言的选项卡：

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

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

async function main() {
  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("locations");

  const result = await collection.aggregate([
    { $project: { item: { $toLower: "$Desk" } } }
  ]).toArray();

  console.log(result);

  await client.close();
}

main();
```

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

```
from pymongo import MongoClient

def main():
    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["locations"]

    result = list(collection.aggregate([
        { "$project": { "item": { "$toLower": "$Desk" } } }
    ]))

    print(result)

    client.close()

if __name__ == "__main__":
    main()
```

------