[{"name":"Test","votes":[{"user":"joe","action":"up"},{"user":"john","action":"up"},{"user":"emma","action":"down"}]}]

db.collection.aggregate([{"$project":{"_id":0,"upDown":{"$reduce":{"input":"$votes","initialValue":[0,0],"in":{"$let":{"vars":{"votes":"$$value","vote":"$$this"},"in":{"$cond":[{"$eq":["$$vote.action","up"]},[{"$add":[{"$arrayElemAt":["$$votes",0]},1]},{"$arrayElemAt":["$$votes",1]}],[{"$arrayElemAt":["$$votes",0]},{"$add":[{"$arrayElemAt":["$$votes",1]},1]}]]}}}}}}},{"$project":{"up":{"$arrayElemAt":["$upDown",0]},"down":{"$arrayElemAt":["$upDown",1]}}}])