[{"_id":1,firstname:"John",lastname:"Doe",credits:[{cDate:"2020-01-16",cAmount:350},{cDate:"2021-02-07",cAmount:180},{cDate:"2021-06-25",cAmount:650},]},{"_id":2,firstname:"Bob",lastname:"Smith",credits:[{cDate:"2020-03-19",cAmount:200},{cDate:"2020-08-20",cAmount:90},{cDate:"2021-11-11",cAmount:300},]}]

db.collection.aggregate([{$match:{"credits.cDate":{$regex:"2021"}}},{$project:{_id:0,firstname:1,lastname:1,total:{$reduce:{input:"$credits",initialValue:0,in:{$cond:[{$eq:[{$substr:["$$this.cDate",0,4]},"2021"]},{$sum:["$$value","$$this.cAmount"]},"$$value"]}}}}}])