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

Amazon DocumentDB 中的`$$DESCEND`运算符是管道阶段中使用的特殊位置数组运算符。`$redact`它指示聚合管道进入当前文档并处理所有字段，无论其嵌套级别如何。

当`$redact`舞台遇到`$$DESCEND`操作员时，它将保持当前文档中的所有字段可见，并在管道中进一步处理它们。当你想根据条件有选择地编辑或修剪某些字段，同时保留文档的结构时，这很有用。

**参数**

无。

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

在此示例中，我们将使用带有`$$DESCEND`运算符的`$redact`舞台来有选择地显示`code`字段等于 “Reg” 的文档。

**创建示例文档**

```
db.patient.insertMany([
  { "_id": 1, "code": "Emp", "patient": "John Doe", "DOB": "1/1/1980", "Hospital": "Main" },
  { "_id": 2, "code": "Reg", "patient": "Jane Doe", "DOB": "3/27/1989", "Hospital": "Cherry Hill" },
  { "_id": 3, "code": "Emp", "patient": "Bob Smith", "DOB": "6/15/1975", "Hospital": "Downtown" }
]);
```

**查询示例**

```
db.patient.aggregate([
  { $redact: {
    $cond: {
      if: { $eq: ["Reg", "$code"] },
      then: "$$DESCEND",
      else: "$$PRUNE"
    }
  }}
]);
```

**输出**

```
{
  "_id": 2,
  "code": "Reg",
  "patient": "Jane Doe",
  "DOB": "3/27/1989",
  "Hospital": "Cherry Hill"
}
```

## 代码示例
<a name="DESCEND-code"></a>

要查看使用该`$$DESCEND`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 redactPatients() {
  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('patient');

  const result = await collection.aggregate([
    { $redact: {
      $cond: {
        if: { $eq: ["Reg", "$code"] },
        then: "$$DESCEND",
        else: "$$PRUNE"
      }
    }}
  ]).toArray();

  console.log(result);
  client.close();
}

redactPatients();
```

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

```
from pymongo import MongoClient

def redact_patients():
    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.patient

    result = list(collection.aggregate([
        { "$redact": {
            "$cond": {
                "if": { "$eq": ["Reg", "$code"] },
                "then": "$$DESCEND",
                "else": "$$PRUNE"
            }
        }}
    ]))

    print(result)
    client.close()

redact_patients()
```

------