Database

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

Query

db.collection.aggregate([{"$project":{"_id":0,"up":{"$size":{"$filter":{"input":"$votes","as":"vote","cond":{"$eq":["$$vote.action","up"]}}}},"down":{"$size":{"$filter":{"input":"$votes","as":"vote","cond":{"$eq":["$$vote.action","down"]}}}}}}])

Result