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$lastN - Amazon DocumentDB
Example (MongoDB Shell)Expression usage example (MongoDB Shell)Code examples

$lastN

New from version 8.0.1.

The $lastN operator in Amazon DocumentDB returns the last N elements. When used as an accumulator in a $group stage, it returns an array of the last N values in each group. When used as an array expression operator, it returns the last N elements of an array.

Parameters

  • input: The expression that resolves to the field or array from which to return values.

  • n: A positive integer that specifies how many values to return. When used as an accumulator in a $group stage, n can also be an expression, as long as it resolves to a positive integer based on the group _id field.

Example (MongoDB Shell)

The following example shows how to use the $lastN accumulator to retrieve the last two quantities for each item during the aggregation.

Note

$lastN selects values in the order that documents reach the $group stage. To return the last N values for a specific ordering (for example, by date or score), add a $sort stage before $group.

Create sample documents

db.sales.insertMany([
  { item: "abc", quantity: 10, date: ISODate("2023-01-01") },
  { item: "abc", quantity: 5, date: ISODate("2023-01-02") },
  { item: "abc", quantity: 8, date: ISODate("2023-01-03") },
  { item: "xyz", quantity: 15, date: ISODate("2023-01-01") },
  { item: "xyz", quantity: 7, date: ISODate("2023-01-02") },
  { item: "xyz", quantity: 3, date: ISODate("2023-01-03") }
]);

Query example

db.sales.aggregate([
  { $group: { _id: "$item", lastTwoQuantities: { $lastN: { input: "$quantity", n: 2 } } } }
]);

Output

[
  { "_id": "abc", "lastTwoQuantities": [5, 8] },
  { "_id": "xyz", "lastTwoQuantities": [7, 3] }
]

Expression usage example (MongoDB Shell)

The $lastN operator can also be used as an expression within a $project stage to return the last N elements of an array field.

Create sample documents

db.inventory.insertMany([
  { _id: 1, item: "abc", tags: ["red", "green", "blue", "yellow", "purple"] },
  { _id: 2, item: "xyz", tags: ["alpha", "beta", "gamma"] }
]);

Query example

db.inventory.aggregate([
  { $project: {
      lastThreeTags: { $lastN: { input: "$tags", n: 3 } }
    }}
]);

Output

[
  { "_id": 1, "lastThreeTags": ["blue", "yellow", "purple"] },
  { "_id": 2, "lastThreeTags": ["alpha", "beta", "gamma"] }
]

Code examples

To view a code example for using the $lastN accumulator, choose the tab for the language that you want to use. The following examples show both accumulator usage (in $group) and expression usage (in $project):

Node.js
const { MongoClient } = require('mongodb');

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

  try {
    await client.connect();
    const db = client.db('test');

    // Accumulator usage: last N values per group
    const sales = db.collection('sales');
    const accumulatorResult = await sales.aggregate([
      { $group: { _id: "$item", lastTwoQuantities: { $lastN: { input: "$quantity", n: 2 } } } }
    ]).toArray();
    console.log('Accumulator result:', accumulatorResult);

    // Expression usage: last N elements of an array field
    const inventory = db.collection('inventory');
    const expressionResult = await inventory.aggregate([
      { $project: { lastThreeTags: { $lastN: { input: "$tags", n: 3 } } } }
    ]).toArray();
    console.log('Expression result:', expressionResult);

  } finally {
    await client.close();
  }
}

example();
Python
from pymongo import MongoClient

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

    try:
        db = client['test']

        # Accumulator usage: last N values per group
        sales = db['sales']
        accumulator_result = list(sales.aggregate([
            { '$group': { '_id': '$item', 'lastTwoQuantities': { '$lastN': { 'input': '$quantity', 'n': 2 } } } }
        ]))
        print('Accumulator result:', accumulator_result)

        # Expression usage: last N elements of an array field
        inventory = db['inventory']
        expression_result = list(inventory.aggregate([
            { '$project': { 'lastThreeTags': { '$lastN': { 'input': '$tags', 'n': 3 } } } }
        ]))
        print('Expression result:', expression_result)

    finally:
        client.close()

example()