Updating a MongoDB document and adding new keys only in the first document?

To update a MongoDB document and add new keys only to the first document in a collection, use the update() method with an empty query filter and $set operator. This targets the first document that matches the query criteria.

Syntax

db.collection.update(
    {},
    { $set: { "newField": "value" } },
    { upsert: true }
);

Sample Data

db.demo162.insertMany([
    { "StudentName": "Chris" },
    { "StudentName": "Bob" },
    { "StudentName": "David" }
]);

{
    "acknowledged": true,
    "insertedIds": [
        ObjectId("5e3684359e4f06af551997c2"),
        ObjectId("5e3684389e4f06af551997c3"),
        ObjectId("5e36843c9e4f06af551997c4")
    ]
}

Example: Add New Field to First Document

Add a StudentAge field with value 23 to the first document ?

db.demo162.update(
    {},
    { $set: { "StudentAge": 23 } },
    { upsert: true }
);

WriteResult({ "nMatched": 1, "nUpserted": 0, "nModified": 1 })

Verify Result

db.demo162.find();

{ "_id": ObjectId("5e3684359e4f06af551997c2"), "StudentName": "Chris", "StudentAge": 23 }
{ "_id": ObjectId("5e3684389e4f06af551997c3"), "StudentName": "Bob" }
{ "_id": ObjectId("5e36843c9e4f06af551997c4"), "StudentName": "David" }

How It Works

• Empty query filter {} matches all documents but update() only modifies the first match by default
• $set operator adds the new field or updates existing field values
• upsert: true creates a new document if no matches are found (optional)

Conclusion

Use update() with an empty query filter and $set to add new fields only to the first document. MongoDB's default behavior updates just the first matching document unless multi: true is specified.