$bottomN - Amazon DocumentDB
Example (MongoDB Shell)Code examples
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$bottomN

New from version 8.0.1.

Use the $bottomN accumulator in the $group stage to return the bottom N elements within a group according to a specified sort order. If a group contains fewer than N elements, $bottomN returns all elements in the group.

Parameters

  • n: A positive integer, or an expression that resolves to one, specifying how many bottom results to return per group.

  • sortBy: A document specifying the sort order. Use 1 for ascending or -1 for descending.

  • output: An expression that specifies the fields to return from each of the bottom N documents.

Example (MongoDB Shell)

The following example demonstrates how to use the $bottomN accumulator to find the bottom 2 sales (lowest quantity) per item in a sales collection.

Create sample documents

db.sales.insertMany([
  { item: "abc", quantity: 10, price: 5 },
  { item: "abc", quantity: 7, price: 8 },
  { item: "abc", quantity: 5, price: 10 },
  { item: "xyz", quantity: 15, price: 3 },
  { item: "xyz", quantity: 9, price: 6 },
  { item: "xyz", quantity: 3, price: 12 }
])

Query example

db.sales.aggregate([
  { $group: { _id: "$item", bottomTwoSales: { $bottomN: { n: 2, sortBy: { quantity: -1 }, output: { quantity: "$quantity", price: "$price" } } } } }
])

Output

[
  { "_id": "xyz", "bottomTwoSales": [{ "quantity": 9, "price": 6 }, { "quantity": 3, "price": 12 }] },
  { "_id": "abc", "bottomTwoSales": [{ "quantity": 7, "price": 8 }, { "quantity": 5, "price": 10 }] }
]

Code examples

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

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

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

  try {
    await client.connect();

    const db = client.db('test');
    const collection = db.collection('sales');

    const result = await collection.aggregate([
      { $group: { _id: "$item", bottomTwoSales: { $bottomN: { n: 2, sortBy: { quantity: -1 }, output: { quantity: "$quantity", price: "$price" } } } } }
    ]).toArray();

    console.log(result);

  } catch (error) {
    console.error('Error:', error);
  } finally {
    await client.close();
  }
}

example();
Python
from pymongo import MongoClient
from pprint import pprint

def example():
    client = None
    try:
        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['sales']

        result = list(collection.aggregate([
            { '$group': { '_id': '$item', 'bottomTwoSales': { '$bottomN': { 'n': 2, 'sortBy': { 'quantity': -1 }, 'output': { 'quantity': '$quantity', 'price': '$price' } } } } }
        ]))

        pprint(result)

    except Exception as e:
        print(f"An error occurred: {e}")

    finally:
        if client:
            client.close()

example()