How to apply a condition only if field exists in MongoDB?

You can use $or operator for this. Let us first create a collection with documents −

> db.applyConditionDemo.insertOne({"StudentName":"Larry","StudentAge":21,"StudentMarks":45});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5cb80b78623186894665ae36")
}
> db.applyConditionDemo.insertOne({"StudentName":"Sam","StudentAge":23,"StudentMarks":55});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5cb80b87623186894665ae37")
}
> db.applyConditionDemo.insertOne({"StudentName":"David","StudentAge":21,"StudentMarks":65});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5cb80b95623186894665ae38")
}
> db.applyConditionDemo.insertOne({"StudentName":"Carol","StudentAge":24,"StudentMarks":78});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5cb80ba3623186894665ae39")
}
> db.applyConditionDemo.insertOne({"StudentName":"Chris","StudentAge":21,"StudentMarks":88});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5cb80bae623186894665ae3a")
}
> db.applyConditionDemo.insertOne({"StudentName":"Robert","StudentMarks":98});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5cb80c3d623186894665ae3b")
}

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

> db.applyConditionDemo.find().pretty();

This will produce the following output −

{
   "_id" : ObjectId("5cb80b78623186894665ae36"),
   "StudentName" : "Larry",
   "StudentAge" : 21,
   "StudentMarks" : 45
}
{
   "_id" : ObjectId("5cb80b87623186894665ae37"),
   "StudentName" : "Sam",
   "StudentAge" : 23,
   "StudentMarks" : 55
}
{
   "_id" : ObjectId("5cb80b95623186894665ae38"),
   "StudentName" : "David",
   "StudentAge" : 21,
   "StudentMarks" : 65
}
{
   "_id" : ObjectId("5cb80ba3623186894665ae39"),
   "StudentName" : "Carol",
   "StudentAge" : 24,
   "StudentMarks" : 78
}
{
   "_id" : ObjectId("5cb80bae623186894665ae3a"),
   "StudentName" : "Chris",
   "StudentAge" : 21,
   "StudentMarks" : 88
}
{
   "_id" : ObjectId("5cb80c3d623186894665ae3b"),
   "StudentName" : "Robert",
   "StudentMarks" : 98
}

Following is the query to apply a condition only if field exists −

> db.applyConditionDemo.find({ $or: [ { StudentAge: { $exists:false } }, { StudentAge:21 } ]}).pretty();

This will produce the following output −

{
   "_id" : ObjectId("5cb80b78623186894665ae36"),
   "StudentName" : "Larry",
   "StudentAge" : 21,
   "StudentMarks" : 45
}
{
   "_id" : ObjectId("5cb80b95623186894665ae38"),
   "StudentName" : "David",
   "StudentAge" : 21,
   "StudentMarks" : 65
}
{
   "_id" : ObjectId("5cb80bae623186894665ae3a"),
   "StudentName" : "Chris",
   "StudentAge" : 21,
   "StudentMarks" : 88
}
{
   "_id" : ObjectId("5cb80c3d623186894665ae3b"),
   "StudentName" : "Robert",
   "StudentMarks" : 98
}