$first - Amazon DocumentDB
Example (MongoDB Shell)Code examples
Services or capabilities described in Amazon Web Services documentation might vary by Region. To see the differences applicable to the China Regions, see Getting Started with Amazon Web Services in China (PDF).

$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()