How to convert ObjectId to string in MongoDB

To convert ObjectId to string in MongoDB, use the $toString operator within an aggregation pipeline. This converts the ObjectId value to its string representation.

Syntax

db.collection.aggregate([
    {
        $project: {
            _id: { $toString: "$_id" },
            // other fields as needed
        }
    }
]);

Sample Data

db.objectidToStringDemo.insertMany([
    { "UserName": "John" },
    { "UserName": "Chris" },
    { "UserName": "Larry" },
    { "UserName": "Robert" }
]);

{
    "acknowledged": true,
    "insertedIds": [
        ObjectId("5c92b80036de59bd9de0639d"),
        ObjectId("5c92b80436de59bd9de0639e"),
        ObjectId("5c92b80936de59bd9de0639f"),
        ObjectId("5c92b81836de59bd9de063a0")
    ]
}

Display Sample Documents

db.objectidToStringDemo.find().pretty();

{ "_id": ObjectId("5c92b80036de59bd9de0639d"), "UserName": "John" }
{ "_id": ObjectId("5c92b80436de59bd9de0639e"), "UserName": "Chris" }
{ "_id": ObjectId("5c92b80936de59bd9de0639f"), "UserName": "Larry" }
{ "_id": ObjectId("5c92b81836de59bd9de063a0"), "UserName": "Robert" }

Convert ObjectId to String

db.objectidToStringDemo.aggregate([
    {
        $project: {
            _id: { $toString: "$_id" },
            UserName: 1
        }
    }
]);

{ "_id": "5c92b80036de59bd9de0639d", "UserName": "John" }
{ "_id": "5c92b80436de59bd9de0639e", "UserName": "Chris" }
{ "_id": "5c92b80936de59bd9de0639f", "UserName": "Larry" }
{ "_id": "5c92b81836de59bd9de063a0", "UserName": "Robert" }

Key Points

• $toString only works within aggregation pipelines, not in regular find() queries.
• Include other fields with fieldName: 1 in the $project stage to preserve them.
• The converted string maintains the same hexadecimal value as the original ObjectId.

Conclusion

Use $toString in an aggregation $project stage to convert ObjectId fields to string format. This is useful for data export, API responses, or string-based operations on ObjectId values.