[{question:"what's the question?",answer:"some answer",points:10},{question:"what's the question again?",answer:"some answer again",points:40},{question:"what's that question?",points:30},]

db.collection.aggregate([{"$facet":{"answered":[{$match:{answer:{$exists:true},},},],"unanswered":[{$match:{answer:{$exists:false},},},],}}])