[{Max:{shoping_list:{food:{Pizza:2,Ramen:1,Sushi:5}},},John:{shoping_list:{food:{Pizza:2,Ramen:1,Burger:1}},}}]

db.collection.aggregate([{$replaceRoot:{newRoot:{foods:{$reduce:{input:{$objectToArray:"$$ROOT"},initialValue:[],in:{$concatArrays:["$$value",{$cond:{if:{$ne:["$$this.k","_id"]},then:{$objectToArray:"$$this.v.shoping_list.food"},else:[]}}]}}}}}},{$unwind:"$foods"},{$group:{_id:"$foods.k",sum:{$sum:"$foods.v"}}}])