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$addToSet

The $addToSet aggregation operator returns an array of unique values from a specified expression for each group. It is used within the $group stage to accumulate distinct values, automatically eliminating duplicates.

Parameters

Example (MongoDB Shell)

The following example demonstrates using the $addToSet operator to collect unique cities where orders were placed for each customer.

Create sample documents

db.orders.insertMany([
  { _id: 1, customer: "Alice", city: "Seattle", amount: 100 },
  { _id: 2, customer: "Alice", city: "Portland", amount: 150 },
  { _id: 3, customer: "Bob", city: "Seattle", amount: 200 },
  { _id: 4, customer: "Alice", city: "Seattle", amount: 75 },
  { _id: 5, customer: "Bob", city: "Boston", amount: 300 }
]);

Query example

db.orders.aggregate([
  {
    $group: {
      _id: "$customer",
      cities: { $addToSet: "$city" }
    }
  }
]);

Output

[
  { _id: 'Bob', cities: [ 'Seattle', 'Boston' ] },
  { _id: 'Alice', cities: [ 'Seattle', 'Portland' ] }
]

Code examples

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

Node.js

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

async function example() {
  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('orders');

  const result = await collection.aggregate([
    {
      $group: {
        _id: "$customer",
        cities: { $addToSet: "$city" }
      }
    }
  ]).toArray();

  console.log(result);
  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')
    db = client['test']
    collection = db['orders']

    result = list(collection.aggregate([
        {
            '$group': {
                '_id': '$customer',
                'cities': { '$addToSet': '$city' }
            }
        }
    ]))

    print(result)
    client.close()

example()