[{"_id":"foo1","1":{"bar":6},"0":{"bar":11},"3":{"bar":8},"2":{"bar":0},"5":{"bar":8},"4":{"bar":19},"6":{"bar":8}},{"_id":"foo2","1":{"bar":18},"0":{"bar":3},"3":{"bar":19},"2":{"bar":0},"5":{"bar":13},"4":{"bar":17},"7":{"bar":8},"6":{"bar":8},"8":{"bar":8}},{"_id":"foo3","1":{"bar":0},"0":{"bar":2},"3":{"bar":18},"2":{"bar":2},"4":{"bar":12}}]

db.collection.aggregate([{$project:{_id:0}},{$project:{data:{$objectToArray:"$$ROOT"}}},{$unwind:"$data"},{$group:{_id:"$data.k",bar:{$sum:"$data.v.bar"}}},{$project:{_id:0,data:{$arrayToObject:[[{"k":{$concat:["bar","$_id"]},"v":"$bar"}]]}}},{$replaceRoot:{newRoot:"$data"}}])