db={"trannsactions":[{"_id":"1","Name":"Company 1","ISIN":"123456789111","transactionAmount":"14000"},{"_id":"2","Name":"Company 1","ISIN":"123456789111","transactionAmount":"180000"},{"_id":"3","Name":"Company 2","ISIN":"123456789115","transactionAmount":"23400"},{"_id":"4","Name":"Company 3","ISIN":"123456789117","transactionAmount":"1000"}],"companies":[{"_id":"c1","Name_short":"Comp 1","ISIN":"123456789111"},{"_id":"c2","Name_short":"Comp 2","ISIN":"123456789115"},{"_id":"c3","Name_short":"Comp 3","ISIN":"123456789117"},{"_id":"c4","Name_short":"Comp 4","ISIN":"123456789118"}]}

db.companies.aggregate([{$lookup:{from:"trannsactions",localField:"ISIN",foreignField:"ISIN",as:"matchedTransactions"}},{$project:{_id:1,name:"$Name_short",ISIN:"$ISIN",transactionAmount:{$sum:{$map:{input:"$matchedTransactions.transactionAmount",in:{$toDouble:"$$this"}}}}}}])