MongoDB query to return specific fields from an array?

To return specific fields from an array in MongoDB, use the aggregation framework with $project stage. You can extract individual array elements or transform array data into specific output formats.

Syntax

db.collection.aggregate([
    {
        $project: {
            fieldName: { $arrayElemAt: ["$arrayField.subField", index] },
            "arrayField.subField": 1
        }
    }
]);

Sample Data

db.returnSpecificFieldDemo.insertOne({
    "StudentId": 1,
    "StudentDetails": [
        {
            "StudentName": "Larry",
            "StudentAge": 21,
            "StudentCountryName": "US"
        },
        {
            "StudentName": "Chris",
            "StudentAge": 23,
            "StudentCountryName": "AUS"
        }
    ]
});

{
    "acknowledged": true,
    "insertedId": ObjectId("5ce23d3236e8b255a5eee943")
}

Example 1: Extract Specific Array Element

Return StudentId and the country name from the second student (index 1) ?

db.returnSpecificFieldDemo.aggregate([
    {
        $project: {
            _id: 0,
            StudentId: "$StudentId",
            StudentCountryName: { 
                $arrayElemAt: ["$StudentDetails.StudentCountryName", 1] 
            }
        }
    }
]);

{ "StudentId": 1, "StudentCountryName": "AUS" }

Example 2: Return Multiple Array Fields

Project only StudentName and StudentAge from all array elements ?

db.returnSpecificFieldDemo.aggregate([
    {
        $project: {
            _id: 0,
            StudentId: 1,
            "StudentDetails.StudentName": 1,
            "StudentDetails.StudentAge": 1
        }
    }
]);

{
    "StudentId": 1,
    "StudentDetails": [
        { "StudentName": "Larry", "StudentAge": 21 },
        { "StudentName": "Chris", "StudentAge": 23 }
    ]
}

Key Points

• $arrayElemAt extracts a single element from an array by index position.
• Dot notation like "arrayField.subField" projects specific fields from all array elements.
• Use _id: 0 to exclude the default ObjectId from results.

Conclusion

MongoDB's aggregation $project stage allows precise field selection from arrays. Use $arrayElemAt for single elements or dot notation for filtering fields across all array elements.