How to use $ifNull with MongoDB aggregation?

The $ifNull evaluates an expression and returns the value of the expression if the expression evaluates to a non-null value.

Let us first create a collection with documents −

> db.demo372.insertOne({"FirstName":"Chris"});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5e591aea2ae06a1609a00af6")
}
> db.demo372.insertOne({"FirstName":null});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5e591aef2ae06a1609a00af7")
}
> db.demo372.insertOne({"FirstName":"David"});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5e591af42ae06a1609a00af8")
}
> db.demo372.insertOne({"FirstName":null});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5e591afb2ae06a1609a00af9")
}

Display all documents from a collection with the help of find() method −

> db.demo372.find();

This will produce the following output −

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

Following is the query to use $ifNull with aggregation−

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

This will produce the following output −

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