Database
[{"_id":ObjectId("5f7753db63cce11af6a7146e"),"completedExams":[{"_id":"5f778650cbc02f43e25064e0","createdAt":"2020-10-02T19:58:08.146Z","exam":"Test-1","score":2},{"_id":"5f77868ccbc02f43e25064e2","createdAt":"2020-10-02T19:59:08.493Z","exam":"Test-2","score":1}]}]
Query
db.collection.aggregate([{$match:{_id:ObjectId("5f7753db63cce11af6a7146e")}},{$unwind:"$completedExams"},{$sort:{"completedExams.createdAt":-1}},{"$replaceRoot":{"newRoot":"$completedExams"}}])