[{"result":{"invoices":[{"product":"baseball","amount":4,"tax":1},{"product":"basketball","amount":10,"tax":2}]}}]

db.collection.aggregate([{$set:{"result.invoices":{"$map":{"input":"$result.invoices","as":"i","in":{"$mergeObjects":["$$i",{"total":{$sum:["$$i.amount","$$i.tax"]}}]}}}}}])