[{accuracy:{"low":4}},{accuracy:{"low":6}},{accuracy:{"high":5}},{accuracy:{"medium":2,"high":3}}]

db.collection.aggregate([{$set:{accuracy:{$objectToArray:"$accuracy"}}},{$unwind:"$accuracy"},{$group:{_id:"$accuracy.k",value:{$sum:"$accuracy.v"}}},{$group:{_id:null,accuracy:{$push:{$arrayToObject:[[{k:"$_id",v:"$value"}]]}}}}])