[{account_id:"ORG1",progress:[{week:1,goal:5000,raised:2400},{week:2,goal:5100,raised:1000}]},{account_id:"ORG2",progress:[{week:1,goal:5000,raised:2400},{week:2,goal:5100,raised:1000}]}]

db.collection.update({account_id:"ORG1"},[{$set:{lastIndex:{$subtract:[{$size:"$progress"},1]}}},{$set:{lastIndex:"$$REMOVE",progress:{$reduce:{input:"$progress",initialValue:[],in:{$concatArrays:["$$value",[{$cond:[{$eq:[{$size:"$$value"},"$lastIndex"]},{$mergeObjects:["$$this",{raised:{$add:["$$this.raised",77]}}]},"$$this"]}]]}}}}}])