[{userId:"",orderNumber:"",items:[{_id:"1",rateInfo:{baseRate:100,tolls:50,fees:25,},},{_id:"2",rateInfo:{baseRate:200,tolls:25,fees:10,},},{_id:"3",rateInfo:{baseRate:25,tolls:5,fees:0,},},],}]

db.collection.aggregate([{"$addFields":{"items":{"$map":{"input":"$items","as":"i","in":{_id:"$$i._id",rateInfo:{baseRate:"$$i.rateInfo.baseRate",tolls:"$$i.rateInfo.tolls",fees:"$$i.rateInfo.fees"},rateTotal:{"$add":["$$i.rateInfo.baseRate","$$i.rateInfo.tolls","$$i.rateInfo.fees"]}}}}}}])