[{"caseNumber":"2020921008981","salesOrder":{"purchaseItems":[{"itemRefId":"5b55ac7f0550de00210a3b24","purchaseQty":NumberInt(1),},{"itemRefId":"5b55ac7f0550de00210a3b25","purchaseQty":NumberInt(2),}],"items":[{"_id":ObjectId("5b55ac7f0550de00210a3b24"),baseUom:"TABLET"},{"_id":ObjectId("5b55ac7f0550de00210a3b25"),baseUom:"VACCINE"}]}}]

db.collection.aggregate([{$project:{itemQtyList:{$reduce:{input:{$range:[0,{$size:"$salesOrder.purchaseItems"}]},initialValue:"",in:{$concat:["$$value",{$cond:[{$eq:["$$this",0]},""," \n "]},{$toString:{$arrayElemAt:["$salesOrder.purchaseItems.purchaseQty","$$this"]}}," ",{$arrayElemAt:["$salesOrder.items.baseUom","$$this"]}]}}}}}])