How to prepend string to entire column in MongoDB?

To prepend a string to an entire column in MongoDB, use the $concat operator within the aggregation framework. The $concat operator combines the prepend string with the existing field value.

Syntax

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

Sample Data

db.prependDemo.insertMany([
    {"StudentFirstName": "John"},
    {"StudentFirstName": "Chris"},
    {"StudentFirstName": "Robert"}
]);

{
    "acknowledged": true,
    "insertedIds": [
        ObjectId("5ccf3bcedceb9a92e6aa1955"),
        ObjectId("5ccf3bd3dceb9a92e6aa1956"),
        ObjectId("5ccf3bd8dceb9a92e6aa1957")
    ]
}

Display the documents to verify the data ?

db.prependDemo.find();

{ "_id": ObjectId("5ccf3bcedceb9a92e6aa1955"), "StudentFirstName": "John" }
{ "_id": ObjectId("5ccf3bd3dceb9a92e6aa1956"), "StudentFirstName": "Chris" }
{ "_id": ObjectId("5ccf3bd8dceb9a92e6aa1957"), "StudentFirstName": "Robert" }

Example: Prepend "Mr." to StudentFirstName

db.prependDemo.aggregate([
    {
        $project: {
            "StudentFirstName": {
                $concat: ["Mr.", "$StudentFirstName"]
            }
        }
    }
]);

{ "_id": ObjectId("5ccf3bcedceb9a92e6aa1955"), "StudentFirstName": "Mr.John" }
{ "_id": ObjectId("5ccf3bd3dceb9a92e6aa1956"), "StudentFirstName": "Mr.Chris" }
{ "_id": ObjectId("5ccf3bd8dceb9a92e6aa1957"), "StudentFirstName": "Mr.Robert" }

Key Points

• $concat creates a new string by joining multiple strings in order
• $project stage reshapes documents with the modified field
• Original data remains unchanged − this only transforms the output

Conclusion

Use $concat within $project to prepend strings to column values in MongoDB. This approach preserves original data while displaying transformed results in the aggregation pipeline output.