How to remove duplicate values inside a list in MongoDB?

To remove duplicate values from an array field in MongoDB, use the aggregation framework with the $setUnion operator. This operator creates a set union between the array and an empty array, automatically eliminating duplicates.

Syntax

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

Sample Data

db.removeDuplicatesDemo.insertOne({
    "InstructorName": "Chris",
    "InstructorAge": 34,
    "InstructorSubject": [
        "Java", "C", "Java", "C++", "MongoDB", "MySQL", "MongoDB"
    ]
});

{
    "acknowledged": true,
    "insertedId": ObjectId("5cb9d96c895c4fd159f80807")
}

Verify Original Data

db.removeDuplicatesDemo.find().pretty();

{
    "_id": ObjectId("5cb9d96c895c4fd159f80807"),
    "InstructorName": "Chris",
    "InstructorAge": 34,
    "InstructorSubject": [
        "Java",
        "C",
        "Java",
        "C++",
        "MongoDB",
        "MySQL",
        "MongoDB"
    ]
}

Remove Duplicates from Array

db.removeDuplicatesDemo.aggregate([
    {
        $project: {
            "InstructorName": 1,
            "InstructorAge": 1,
            "InstructorSubject": { $setUnion: ["$InstructorSubject", []] }
        }
    }
]).pretty();

{
    "_id": ObjectId("5cb9d96c895c4fd159f80807"),
    "InstructorName": "Chris",
    "InstructorAge": 34,
    "InstructorSubject": [
        "C",
        "C++",
        "Java",
        "MongoDB",
        "MySQL"
    ]
}

How It Works

The $setUnion operator performs a set union between $InstructorSubject and an empty array []. Since set operations automatically remove duplicates, this effectively deduplicates the original array while preserving unique values in alphabetical order.

Conclusion

Use $setUnion with an empty array in the aggregation pipeline to remove duplicates from array fields. This method is efficient and maintains the unique values in sorted order.