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

`$$PRUNE`系统变量与聚合管道中的`$redact`阶段一起使用，从结果中排除文档或嵌入式文档级别。当条件计算为时`$$PRUNE`，当前文档或子文档将从输出中删除。它通常与`$$DESCEND`（用于保留和遍历文档）或`$$KEEP`（将文档保持在所有级别）一起使用。

**参数**

无。`$$PRUNE`系统变量不带任何参数，必须与一起使用`$redact`。

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

以下示例演示如何`$$PRUNE`使用和将 30 岁`$redact`以上的用户排除在结果之外。

**创建示例文档**

```
db.users.insert([
  { _id:1, name: "Carlos Salazar", age: 35, address: { street: "123 Main St", city: "Anytown", state: "CA" } },
  { _id:2, name: "Saanvi Sarkar", age: 28, address: { street: "456 Oak Rd", city: "Someplace", state: "NY" } },
  { _id:3, name: "Li Juan", age: 42, address: { street: "789 Pine Ave", city: "Springfield", state: "TX" } }
])
```

**查询示例**

```
db.users.aggregate([
  {
    $redact: {
      $cond: {
        if: { $gt: ["$age", 30] },
        then: "$$PRUNE",
        else: "$$DESCEND"
      }
    }
  }
])
```

**输出**

```
[
  {
    "_id": 2,
    "name": "Saanvi Sarkar",
    "age": 28,
    "address": {
      "street": "456 Oak Rd",
      "city": "Someplace",
      "state": "NY"
    }
  }
]
```

## 代码示例
<a name="PRUNE-code"></a>

要查看使用该`$$PRUNE`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');

const client = new MongoClient('mongodb://<username>:<password>@<cluster-endpoint>:27017/?tls=true&tlsCAFile=global-bundle.pem&replicaSet=rs0&readPreference=secondaryPreferred&retryWrites=false');

async function main() {
  await client.connect();
  const db = client.db('test');
  const users = db.collection('users');

  const result = await users.aggregate([
    {
      $redact: {
        $cond: {
          if: { $gt: ["$age", 30] },
          then: "$$PRUNE",
          else: "$$DESCEND"
        }
      }
    }
  ]).toArray();

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

main();
```

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

```
from pymongo import MongoClient

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

result = list(users.aggregate([
    {
        '$redact': {
            '$cond': {
                'if': { '$gt': ['$age', 30] },
                'then': '$$PRUNE',
                'else': '$$DESCEND'
            }
        }
    }
]))

print(result)
client.close()
```

------