[{_id:"134",items:[{_id:"itemId1",quantity:2,price:100,couponsApplied:[]},{_id:"itemId2",quantity:2,price:200,couponsApplied:[]}]}]

db.collection.update({},[{$set:{"items":{$map:{input:"$items",in:{$mergeObjects:["$$this",{couponsApplied:{$concatArrays:["$$this.couponsApplied",[{couponAmount:{$multiply:["$$this.price","$$this.quantity",{$divide:[5,100]}]}}]]}}]}}}}}])