$$PRUNE

The $$PRUNE system variable is used with the $redact stage in the aggregation pipeline to exclude documents or embedded document levels from the results. When a condition evaluates to $$PRUNE, the current document or subdocument is removed from the output. It is typically used with $$DESCEND (to keep and traverse the document) or $$KEEP (to keep the document at all levels).

Parameters

None. The $$PRUNE system variable is used without any parameters and must be used with $redact.

Example (MongoDB Shell)

The following example demonstrates how to use $$PRUNE with $redact to exclude users over 30 years old from the results.

Create sample documents

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" } }
])

Query example

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

Output

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

Code examples

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

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()