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

The $addToSet operator in Amazon DocumentDB is used to add a value to an array only if the value is not already present in the array. This is useful for ensuring that an array contains unique elements.

Parameters

  • field: The field to update.

  • value: The value to add to the array field. This can be a single value or an expression.

Example (MongoDB Shell)

The following example demonstrates how to use the $addToSet operator to add unique elements to an array.

Create sample documents

db.products.insertMany([
  { "_id": 1, "item": "apple", "tags": ["fruit", "red", "round"] },
  { "_id": 2, "item": "banana", "tags": ["fruit", "yellow"] },
  { "_id": 3, "item": "cherry", "tags": ["fruit", "red"] }
])

Query example

db.products.update(
  { "item": "apple" },
  { $addToSet: { "tags": "green" } }
)

Output

{ "_id": 1, "item": "apple", "tags": ["fruit", "red", "round", "green"] }

In this example, the $addToSet operator adds the "green" tag to the "tags" array of the document where the "item" field is "apple". Since "green" was not already in the array, it was added.

Code examples

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

  await collection.updateOne(
    { "item": "apple" },
    { $addToSet: { "tags": "green" } }
  );

  const updatedDoc = await collection.findOne({ "item": "apple" });
  console.log(updatedDoc);

  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.products

    collection.update_one(
        {"item": "apple"},
        {"$addToSet": {"tags": "green"}}
    )

    updated_doc = collection.find_one({"item": "apple"})
    print(updated_doc)

    client.close()

example()