Database

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

Query

db.collection.aggregate([{"$unwind":{"path":"$votes"}},{"$replaceRoot":{"newRoot":{"$cond":[{"$eq":["$votes.action","up"]},{"up":1},{"down":1}]}}},{"$group":{"_id":null,"up":{"$sum":"$up"},"down":{"$sum":"$down"}}},{"$project":{"_id":0}}])

Result