Get the first element in an array and return using MongoDB Aggregate?

To get the first element from an array using MongoDB aggregation, use the $arrayElemAt operator or combine $unwind with $first. The $arrayElemAt method is simpler and more efficient for this task.

Syntax

db.collection.aggregate([
    {
        $project: {
            "fieldName": { $arrayElemAt: ["$arrayField", 0] }
        }
    }
]);

Sample Data

db.getFirstElementInArrayDemo.insertMany([
    {
        "StudentName": "John",
        "StudentSubject": ["MongoDB", "Python", "MySQL"]
    },
    {
        "StudentName": "Chris",
        "StudentSubject": ["Java", "C"]
    },
    {
        "StudentName": "Robert",
        "StudentSubject": ["C++", "Ruby"]
    }
]);

Method 1: Using $arrayElemAt (Recommended)

Get the first element from each array using $arrayElemAt with index 0 ?

db.getFirstElementInArrayDemo.aggregate([
    {
        $project: {
            "_id": 0,
            "StudentName": 1,
            "FirstSubject": { $arrayElemAt: ["$StudentSubject", 0] }
        }
    }
]);

{ "StudentName": "John", "FirstSubject": "MongoDB" }
{ "StudentName": "Chris", "FirstSubject": "Java" }
{ "StudentName": "Robert", "FirstSubject": "C++" }

Method 2: Using $unwind and $first

Alternative approach using $unwind to deconstruct arrays, then $first to get the first element ?

db.getFirstElementInArrayDemo.aggregate([
    { $unwind: "$StudentSubject" },
    {
        $group: {
            "_id": "$_id",
            "StudentName": { $first: "$StudentName" },
            "FirstElement": { $first: "$StudentSubject" }
        }
    },
    { $project: { "_id": 0, "StudentName": 1, "FirstElement": 1 } }
]);

{ "StudentName": "John", "FirstElement": "MongoDB" }
{ "StudentName": "Chris", "FirstElement": "Java" }
{ "StudentName": "Robert", "FirstElement": "C++" }

Alternative: Using $slice in find()

For simple queries without aggregation, use $slice to limit array elements ?

db.getFirstElementInArrayDemo.find(
    {},
    { "StudentSubject": { $slice: 1 } }
);

{ "_id": ObjectId("..."), "StudentName": "John", "StudentSubject": ["MongoDB"] }
{ "_id": ObjectId("..."), "StudentName": "Chris", "StudentSubject": ["Java"] }
{ "_id": ObjectId("..."), "StudentName": "Robert", "StudentSubject": ["C++"] }

Key Points

• $arrayElemAt is the most efficient method for accessing array elements by index.
• $unwind + $first requires more pipeline stages but offers flexibility for complex operations.
• $slice in find() returns the element within an array, not as a single value.

Conclusion

Use $arrayElemAt with index 0 for the simplest and most efficient way to get the first array element. The $unwind approach provides more control when you need additional array processing in your aggregation pipeline.