How to add an extra field in a sub document in MongoDB?

To add an extra field to a subdocument in MongoDB, use the $set operator with dot notation to target specific elements within an array. Use the array index to specify which subdocument to update.

Syntax

db.collection.update(
    { "field": "value" },
    { "$set": { "arrayField.index.newField": "newValue" } }
);

Sample Data

db.addExtraFieldDemo.insertOne({
    "_id": 1,
    "UserName": "Larry",
    "UserOtherDetails": [
        {
            "UserCountryName": "US",
            "UserAge": 23
        },
        {
            "UserCountryName": "UK",
            "UserAge": 24
        }
    ]
});

{ "acknowledged": true, "insertedId": 1 }

Let us display the current document structure ?

db.addExtraFieldDemo.find().pretty();

{
    "_id": 1,
    "UserName": "Larry",
    "UserOtherDetails": [
        {
            "UserCountryName": "US",
            "UserAge": 23
        },
        {
            "UserCountryName": "UK",
            "UserAge": 24
        }
    ]
}

Example: Adding Field to First Subdocument

Add an isMarried field to the first subdocument (index 0) ?

db.addExtraFieldDemo.update(
    { "_id": 1 },
    { "$set": { "UserOtherDetails.0.isMarried": true } }
);

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

Verify the updated document ?

db.addExtraFieldDemo.find().pretty();

{
    "_id": 1,
    "UserName": "Larry",
    "UserOtherDetails": [
        {
            "UserCountryName": "US",
            "UserAge": 23,
            "isMarried": true
        },
        {
            "UserCountryName": "UK",
            "UserAge": 24
        }
    ]
}

Key Points

• Use dot notation with array index: "arrayField.index.newField"
• The $set operator adds new fields or updates existing ones
• Array indices start from 0 (first element is index 0)

Conclusion

Use $set with dot notation and array indices to add fields to specific subdocuments. This approach allows precise targeting of array elements without affecting other subdocuments in the same array.