How to use $ifNull with MongoDB aggregation?

The $ifNull operator evaluates an expression and returns the value of the expression if it evaluates to a non-null value. If the expression is null or missing, it returns a specified default value.

Syntax

{
    $ifNull: [ <expression>, <replacement-expression-if-null> ]
}

Sample Data

Let us create a collection with documents containing null values ?

db.demo372.insertMany([
    { "FirstName": "Chris" },
    { "FirstName": null },
    { "FirstName": "David" },
    { "FirstName": null }
]);

{
    "acknowledged": true,
    "insertedIds": [
        ObjectId("5e591aea2ae06a1609a00af6"),
        ObjectId("5e591aef2ae06a1609a00af7"),
        ObjectId("5e591af42ae06a1609a00af8"),
        ObjectId("5e591afb2ae06a1609a00af9")
    ]
}

Display all documents from the collection ?

db.demo372.find();

{ "_id": ObjectId("5e591aea2ae06a1609a00af6"), "FirstName": "Chris" }
{ "_id": ObjectId("5e591aef2ae06a1609a00af7"), "FirstName": null }
{ "_id": ObjectId("5e591af42ae06a1609a00af8"), "FirstName": "David" }
{ "_id": ObjectId("5e591afb2ae06a1609a00af9"), "FirstName": null }

Example: Replace Null Values

Use $ifNull to replace null FirstName values with "NOT PROVIDED" ?

db.demo372.aggregate([
    {
        $project: {
            FirstName: { $ifNull: [ "$FirstName", "NOT PROVIDED" ] }
        }
    }
]);

{ "_id": ObjectId("5e591aea2ae06a1609a00af6"), "FirstName": "Chris" }
{ "_id": ObjectId("5e591aef2ae06a1609a00af7"), "FirstName": "NOT PROVIDED" }
{ "_id": ObjectId("5e591af42ae06a1609a00af8"), "FirstName": "David" }
{ "_id": ObjectId("5e591afb2ae06a1609a00af9"), "FirstName": "NOT PROVIDED" }

Key Points

• $ifNull works with both null values and missing fields
• The replacement value can be a string, number, or any valid expression
• Commonly used in $project stages for data transformation

Conclusion

The $ifNull operator is essential for handling null values in MongoDB aggregation pipelines. It provides a clean way to substitute default values for null or missing fields during data processing.