Push new key element into subdocument of MongoDB?

To add new fields to a subdocument in MongoDB, use the $set operator with dot notation to target the specific subdocument field.

Syntax

db.collectionName.update(
    {"_id": ObjectId("yourObjectId")},
    {$set: {"outerField.newFieldName": "value"}}
);

Sample Data

Let us create a collection with a document containing an empty subdocument ?

db.pushNewKeyDemo.insertOne({
    "UserId": 100,
    "UserDetails": {}
});

{
    "acknowledged": true,
    "insertedId": ObjectId("5cda58f5b50a6c6dd317adbf")
}

View the current document ?

db.pushNewKeyDemo.find();

{"_id": ObjectId("5cda58f5b50a6c6dd317adbf"), "UserId": 100, "UserDetails": {}}

Example: Adding New Field to Subdocument

Add a "UserName" field to the UserDetails subdocument ?

db.pushNewKeyDemo.update(
    {"_id": ObjectId("5cda58f5b50a6c6dd317adbf")},
    {$set: {"UserDetails.UserName": "David Miller"}}
);

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

Verify the updated document ?

db.pushNewKeyDemo.find();

{"_id": ObjectId("5cda58f5b50a6c6dd317adbf"), "UserId": 100, "UserDetails": {"UserName": "David Miller"}}

Adding Multiple Fields

You can add multiple fields to a subdocument in a single operation ?

db.pushNewKeyDemo.update(
    {"_id": ObjectId("5cda58f5b50a6c6dd317adbf")},
    {$set: {
        "UserDetails.Age": 30,
        "UserDetails.City": "New York"
    }}
);

Conclusion

Use $set with dot notation to add new fields to subdocuments. The dot notation format "parentField.newField" allows precise targeting of subdocument properties without affecting other fields.