How do I add a field to existing record in MongoDB?

You can use the update command with the $set operator to add a field to existing records in MongoDB. This operation can add fields to specific documents or to all documents in a collection.

Syntax

db.collection.update(
    { query },
    { $set: { "newFieldName": value } },
    { multi: true }
);

Sample Data

db.addAFieldToEveryRecordDemo.insertMany([
    { "ClientName": "Chris", "ClientAge": 34 },
    { "ClientName": "Robert", "ClientAge": 36 }
]);

{
    "acknowledged": true,
    "insertedIds": [
        ObjectId("5cd00e32588d4a6447b2e061"),
        ObjectId("5cd00e59588d4a6447b2e062")
    ]
}

Display Current Documents

db.addAFieldToEveryRecordDemo.find().pretty();

{
    "_id": ObjectId("5cd00e32588d4a6447b2e061"),
    "ClientName": "Chris",
    "ClientAge": 34
}
{
    "_id": ObjectId("5cd00e59588d4a6447b2e062"),
    "ClientName": "Robert",
    "ClientAge": 36
}

Example: Add Field to All Records

Add a nested field "ClientDetails.ClientCountryName" to all documents ?

db.addAFieldToEveryRecordDemo.update(
    {},
    { $set: { "ClientDetails.ClientCountryName": "" } },
    { multi: true }
);

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

Verify Results

db.addAFieldToEveryRecordDemo.find().pretty();

{
    "_id": ObjectId("5cd00e32588d4a6447b2e061"),
    "ClientName": "Chris",
    "ClientAge": 34,
    "ClientDetails": {
        "ClientCountryName": ""
    }
}
{
    "_id": ObjectId("5cd00e59588d4a6447b2e062"),
    "ClientName": "Robert",
    "ClientAge": 36,
    "ClientDetails": {
        "ClientCountryName": ""
    }
}

Key Points

• Empty query {} matches all documents in the collection.
• { multi: true } ensures the update applies to all matching documents.
• Dot notation creates nested fields automatically if they don't exist.

Conclusion

Use $set with an empty query and multi: true to add fields to all existing records. The operation creates nested structures automatically when using dot notation.