db={"poll":[{"_id":100,"choices":[{"name":"choiceA","id":0,"l":"k"},{"name":"choiceB","id":1,"l":"j"},{"name":"choiceC","id":2,"l":"l"}]}],}

db.poll.aggregate([{"$match":{"_id":100}},{$project:{numberOfVotes:{$gt:[{$size:{$filter:{input:"$choices",as:"choice",cond:{$and:[{$eq:["$$choice.id",2]},{$eq:["$$choice.l","j"]}]}}}},0]}}}])