Implement a MongoDB $cond field in a projection pipeline based on the presence or absence of a field?

For this, you can use $cond along with $anyElementTrue. NULL values (absence of a field) would evaluate to FALSE. With that, an empty array also returns FALSE with $ anyElementTrue.

Let us first create a collection with documents −

> db.presenceDemo.insertOne({"StudentName":null});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5e06f70c25ddae1f53b621f3")
}
> db.presenceDemo.insertOne({"StudentName":"Chris"});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5e06f71425ddae1f53b621f4")
}
> db.presenceDemo.insertOne({"StudentName":null});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5e06f71825ddae1f53b621f5")
}
> db.presenceDemo.insertOne({"StudentName":"David"});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5e06f71e25ddae1f53b621f6")
}

Following is the query to display all documents from a collection with the help of find() method −

> db.presenceDemo.find();

This will produce the following output −

{ "_id" : ObjectId("5e06f70c25ddae1f53b621f3"), "StudentName" : null }
{ "_id" : ObjectId("5e06f71425ddae1f53b621f4"), "StudentName" : "Chris" }
{ "_id" : ObjectId("5e06f71825ddae1f53b621f5"), "StudentName" : null }
{ "_id" : ObjectId("5e06f71e25ddae1f53b621f6"), "StudentName" : "David" }

Here is the query to implement a $cond field based on the presence or absence of a field −

> db.presenceDemo.aggregate([
...    { "$project": {
...       "MyValue": {
...          "$cond": [
...             { "$anyElementTrue": [ [ "$StudentName" ] ] },
...                1,
...                0
...          ]
...       }
...    }}
... ]);

This will produce the following output −

{ "_id" : ObjectId("5e06f70c25ddae1f53b621f3"), "MyValue" : 0 }
{ "_id" : ObjectId("5e06f71425ddae1f53b621f4"), "MyValue" : 1 }
{ "_id" : ObjectId("5e06f71825ddae1f53b621f5"), "MyValue" : 0 }
{ "_id" : ObjectId("5e06f71e25ddae1f53b621f6"), "MyValue" : 1 }