$slice

The $slice update operator modifies an array by limiting its size. When used with the $push operator, it restricts the number of elements in an array, keeping only the specified number of most recent or oldest elements.

Parameters

• field: The array field to modify.

• count: Maximum number of elements to keep. Positive values keep the first N elements, negative values keep the last N elements.

Example (MongoDB Shell)

The following example demonstrates how to use the $slice update operator with $push to maintain a fixed-size array of recent scores.

Create sample documents

db.students.insertOne({
  _id: 1,
  name: "Alice",
  scores: [85, 90, 78]
});

Query example

db.students.updateOne(
  { _id: 1 },
  {
    $push: {
      scores: {
        $each: [92, 88],
        $slice: -3
      }
    }
  }
)

Output

{
  "_id" : 1,
  "name" : "Alice",
  "scores" : [ 78, 92, 88 ]
}

In this example, the $slice: -3 modifier keeps only the last three elements after pushing new values to the array.

Code examples

To view a code example for using the $slice update operator, choose the tab for the language that you want to use:

Node.js

const { MongoClient } = require('mongodb');

async function updateDocument() {
  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('students');

  await collection.updateOne(
    { _id: 1 },
    {
      $push: {
        scores: {
          $each: [92, 88],
          $slice: -3
        }
      }
    }
  );

  const updatedDocument = await collection.findOne({ _id: 1 });
  console.log(updatedDocument);

  await client.close();
}

updateDocument();

Python

from pymongo import MongoClient

def update_document():
    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.students

    collection.update_one(
        {'_id': 1},
        {
            '$push': {
                'scores': {
                    '$each': [92, 88],
                    '$slice': -3
                }
            }
        }
    )

    updated_document = collection.find_one({'_id': 1})
    print(updated_document)

    client.close()

update_document()