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

The `$$DESCEND` operator in Amazon DocumentDB is a special positional array operator used within the `$redact` pipeline stage. It instructs the aggregation pipeline to descend into the current document and process all fields, regardless of their nesting level.

When the `$redact` stage encounters the `$$DESCEND` operator, it will keep all the fields in the current document visible and process them further down the pipeline. This is useful when you want to selectively redact or prune certain fields based on a condition, while retaining the structure of the document.

**Parameters**

None.

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

In this example, we'll use the `$redact` stage with the `$$DESCEND` operator to selectively display documents where the `code` field is equal to "Reg".

**Create sample documents**

```
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" }
]);
```

**Query example**

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

**Output**

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

## Code examples
<a name="DESCEND-code"></a>

To view a code example for using the `$$DESCEND` command, choose the tab for the language that you want to use:

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

------