[{items:[{amount:10,type:"product1",date:"2022-10-05",totalAmount:10},{amount:15,type:"product2",date:"2022-10-07",totalAmount:25},{amount:100,type:"product1",date:"2022-10-10",totalAmount:125}]}]

db.collection.aggregate([{$project:{items:{$reduce:{input:"$items",initialValue:[],in:{$concatArrays:["$$value",[{"$mergeObjects":["$$this",{totalAmount:{$add:["$$this.amount",{$sum:"$$value.amount"}]}}]}]]}}}}}])