db={"currentBills":[{memberId:1,name:"Mark",status:"paid"},{memberId:2,name:"Steve",status:"unpaid"},{memberId:3,name:"William",status:"paid"}],"pastBills":[{memberId:5,name:"Joe",status:"paid"},{memberId:8,name:"Carlos",status:"paid"},{memberId:25,name:"ben",status:"unpaid"}]}

db.currentBills.aggregate([{$group:{_id:"$status",members:{$push:"$$ROOT"}}},{$group:{_id:null,status:{$push:{k:"$_id",v:"$members"}}}},{$replaceRoot:{newRoot:{$arrayToObject:"$status"}}}])