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:{$and:[{$in:[2,"$choices.id"]},{$in:["j","$choices.l"]}]},}}])