[{"name":"Wasim","completedExams":[{exam:"test-1",score:110},{exam:"test-2",score:80}]},{"name":"Khan","completedExams":[{exam:"test-1",score:100},{exam:"test-2",score:90}]}]

db.collection.aggregate([{"$unwind":"$completedExams"},{"$match":{"completedExams.exam":"test-1"}},{"$sort":{"completedExams.score":-1}}])