How can I update all elements in an array with a prefix string?

To update all elements in an array with a prefix string in MongoDB, use forEach() combined with map() to transform each array element and $set to update the array. This approach processes each document individually and applies the prefix to all array elements.

Syntax

db.collection.find().forEach(function (doc) {
    var prefixedArray = doc.arrayField.map(function (element) {
        return "PREFIX" + element;
    });
    db.collection.update(
        {_id: doc._id},
        {$set: {arrayField: prefixedArray}}
    );
});

Sample Data

db.replaceAllElementsWithPrefixDemo.insertMany([
    {
        "StudentNames": [
            "John",
            "Carol"
        ]
    },
    {
        "StudentNames": [
            "Sam"
        ]
    }
]);

{
    "acknowledged": true,
    "insertedIds": [
        ObjectId("5cd91908b50a6c6dd317ad8e"),
        ObjectId("5cd9191cb50a6c6dd317ad8f")
    ]
}

Example: Add "MR." Prefix

Transform all student names by adding the prefix "MR." ?

db.replaceAllElementsWithPrefixDemo.find().forEach(function (myDocumentValue) {
    var prefixValue = myDocumentValue.StudentNames.map(function (myValue) {
        return "MR." + myValue;
    });
    db.replaceAllElementsWithPrefixDemo.update(
        {_id: myDocumentValue._id},
        {$set: {StudentNames: prefixValue}}
    );
});

Verify Result

db.replaceAllElementsWithPrefixDemo.find().pretty();

{
    "_id": ObjectId("5cd91908b50a6c6dd317ad8e"),
    "StudentNames": [
        "MR.John",
        "MR.Carol"
    ]
}
{
    "_id": ObjectId("5cd9191cb50a6c6dd317ad8f"),
    "StudentNames": [
        "MR.Sam"
    ]
}

How It Works

• forEach() iterates through each document in the collection
• map() transforms each array element by applying the prefix
• $set replaces the entire array with the prefixed version

Conclusion

Use forEach() with map() to add prefixes to all array elements efficiently. This approach processes documents individually and replaces the entire array with transformed values in a single update operation.