Aggregate a $slice to get an element in exact position from a nested array in MongoDB?

To get an element from an exact position in a nested array using MongoDB's aggregation framework, use the $slice operator within a $project stage to extract specific elements by their index position.

Syntax

db.collection.aggregate([
    {
        $project: {
            "arrayField": { $slice: ["$arrayField", startIndex, numberOfElements] }
        }
    }
]);

Sample Data

db.exactPositionDemo.insertOne({
    "StudentName": "John",
    "StudentScores": [78, 98, 56, 45, 89]
});

{
    "acknowledged": true,
    "insertedId": ObjectId("5cd29a1c345990cee87fd883")
}

Let's verify our sample data ?

db.exactPositionDemo.find().pretty();

{
    "_id": ObjectId("5cd29a1c345990cee87fd883"),
    "StudentName": "John",
    "StudentScores": [78, 98, 56, 45, 89]
}

Case 1: Get Element at Index 0

Extract the first element (index 0) from StudentScores array ?

db.exactPositionDemo.aggregate([
    {
        $project: {
            "StudentScores": { $slice: ["$StudentScores", 0, 1] }
        }
    }
]);

{ "_id": ObjectId("5cd29a1c345990cee87fd883"), "StudentScores": [78] }

Case 2: Get Element at Index 1

Extract the second element (index 1) from StudentScores array ?

db.exactPositionDemo.aggregate([
    {
        $project: {
            "StudentScores": { $slice: ["$StudentScores", 1, 1] }
        }
    }
]);

{ "_id": ObjectId("5cd29a1c345990cee87fd883"), "StudentScores": [98] }

Key Points

• $slice takes three parameters: [array, startIndex, numberOfElements]
• StartIndex is zero−based (0 = first element, 1 = second element)
• Setting numberOfElements to 1 returns exactly one element at the specified position
• The result is always returned as an array, even for a single element

Conclusion

The $slice operator in aggregation pipelines provides precise control over array element extraction. Use startIndex to specify position and numberOfElements to control how many elements to return from that position.