本文為英文版的機器翻譯版本，如內容有任何歧義或不一致之處，概以英文版為準。

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

Amazon DocumentDB 中的`$$DESCEND`運算子是在`$redact`管道階段中使用的特殊位置陣列運算子。它會指示彙總管道遞減至目前的文件並處理所有欄位，無論其巢狀層級為何。

當`$redact`階段遇到`$$DESCEND`運算子時，它會讓目前文件中的所有欄位保持可見，並進一步處理它們。當您想要根據條件選擇性地修訂或剪除特定欄位，同時保留文件的結構時，這非常有用。

**參數**

無.

## 範例 (MongoDB Shell)
<a name="DESCEND-examples"></a>

在此範例中，我們將使用 `$redact`階段搭配 `$$DESCEND`運算子，選擇性地顯示 `code` 欄位等於 "Reg" 的文件。

**建立範例文件**

```
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" }
]);
```

**查詢範例**

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

**輸出**

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

## 程式碼範例
<a name="DESCEND-code"></a>

若要檢視使用 `$$DESCEND`命令的程式碼範例，請選擇您要使用的語言標籤：

------
#### [ 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()
```

------