db.students.aggregate([ { $project: { final: 1, midterm: 1, extraMarks: { $ifNull: [ "$extraMarks", 0 ] } } }, { $project: { examTotal: { $add: [ "$final", "$midterm","$extraMarks" ] } } } ])
db.students.aggregate([ { $project: { final: 1, midterm: 1, extraMarks: { $ifNull: [ "$extraMarks", 0 ] } } }, { $project: { examTotal: { $add: [ "$final", "$midterm","$extraMarks" ] } } } ])