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

$first

New from version 5.0.

Not supported by Elastic cluster.

The $first operator in Amazon DocumentDB returns the first document from a grouped set of documents. It is commonly used in aggregation pipelines to retrieve the first document that matches a specific condition.

Parameters

  • expression: The expression to return as the first value in each group.

Example (MongoDB Shell)

The following example demonstrates the use of the $first operator to retrieve the first item value encountered for each category during the aggregation.

Note: $first returns the first document based on the current order of documents in the pipeline. To ensure a specific order (e.g., by date, price, etc.), a $sort stage should be used before the $group stage.

Create sample documents

db.products.insertMany([
  { _id: 1, item: "abc", price: 10, category: "food" },
  { _id: 2, item: "jkl", price: 20, category: "food" },
  { _id: 3, item: "xyz", price: 5, category: "toy" },
  { _id: 4, item: "abc", price: 5, category: "toy" }
]);

Query example

db.products.aggregate([
  { $group: { _id: "$category", firstItem: { $first: "$item" } } }
]);

Output

[
  { "_id" : "food", "firstItem" : "abc" },
  { "_id" : "toy", "firstItem" : "xyz" }
]

Code examples

To view a code example for using the $first command, 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('products');

    const result = await collection.aggregate([
      { $group: { _id: "$category", firstItem: { $first: "$item" } } }
    ]).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['products']

        result = list(collection.aggregate([
            { '$group': { '_id': '$category', 'firstItem': { '$first': '$item' } } }
        ]))

        pprint(result)

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

    finally:
        if client:
            client.close()

example()