Insert MongoDB document field only when it's missing?

To insert a MongoDB document field only when it's missing, use the $exists operator in the query condition to target documents where the field doesn't exist, then apply $set to add the field only to those documents.

Syntax

db.collection.update(
    { "fieldName": { "$exists": false } },
    { "$set": { "fieldName": "value" } },
    { "multi": true }
);

Sample Data

db.missingDocumentDemo.insertMany([
    {"StudentFirstName": "Adam", "StudentLastName": "Smith"},
    {"StudentFirstName": "Carol", "StudentLastName": "Taylor"},
    {"StudentFirstName": "David", "StudentLastName": "Miller", "StudentAge": 21}
]);

{
    "acknowledged": true,
    "insertedIds": [
        ObjectId("5cd3fb1eedc6604c74817ce6"),
        ObjectId("5cd3fb29edc6604c74817ce7"),
        ObjectId("5cd3fb40edc6604c74817ce8")
    ]
}

Initial Documents

db.missingDocumentDemo.find();

{
    "_id": ObjectId("5cd3fb1eedc6604c74817ce6"),
    "StudentFirstName": "Adam",
    "StudentLastName": "Smith"
}
{
    "_id": ObjectId("5cd3fb29edc6604c74817ce7"),
    "StudentFirstName": "Carol",
    "StudentLastName": "Taylor"
}
{
    "_id": ObjectId("5cd3fb40edc6604c74817ce8"),
    "StudentFirstName": "David",
    "StudentLastName": "Miller",
    "StudentAge": 21
}

Example: Add Missing StudentAge Field

Add StudentAge field only to documents that don't already have it ?

db.missingDocumentDemo.update(
    { "StudentAge": { "$exists": false } },
    { "$set": { "StudentAge": 23 } },
    { "multi": true }
);

WriteResult({ "nMatched": 2, "nUpserted": 0, "nModified": 2 })

Verify Results

db.missingDocumentDemo.find();

{
    "_id": ObjectId("5cd3fb1eedc6604c74817ce6"),
    "StudentFirstName": "Adam",
    "StudentLastName": "Smith",
    "StudentAge": 23
}
{
    "_id": ObjectId("5cd3fb29edc6604c74817ce7"),
    "StudentFirstName": "Carol",
    "StudentLastName": "Taylor",
    "StudentAge": 23
}
{
    "_id": ObjectId("5cd3fb40edc6604c74817ce8"),
    "StudentFirstName": "David",
    "StudentLastName": "Miller",
    "StudentAge": 21
}

Key Points

• {"fieldName": {"$exists": false}} matches documents missing the specified field.
• {"multi": true} ensures all matching documents are updated, not just the first one.
• Existing field values remain unchanged when the field already exists.

Conclusion

Use $exists: false to selectively add fields only to documents that lack them. This approach preserves existing data while filling gaps in your document structure efficiently.