db={"purchaseorder":[{"_id":"5f0104a8d0c3a06fc9c06625","purchaseItems":[{"product":"5eff29e9e2708a0ca980762e","quantity":1},{"product":"5eff29e9e2708a0ca980762e","quantity":2}],"totalQuantity":0,"documentDate":{"$date":"2020-07-04T16:00:00.000Z"}}],"product":[{"_id":"5eff29e9e2708a0ca980762e","name":"name","code":"code","cost":"1","srp":"1","minimum":"1","startEffectiveDate":{"$date":"2020-07-04T16:00:00.000Z"}}]}

db.purchaseorder.aggregate([{"$lookup":{"from":"product","localField":"purchaseItems.product","foreignField":"_id","as":"product"}},{$project:{documentDate:1,totalQuantity:1,purchaseItems:{$map:{input:"$purchaseItems",as:"item",in:{product:{$arrayElemAt:[{$filter:{input:"$product",as:"prod",cond:{$eq:["$$prod._id","$$item.product"]}}},0]},quantity:"$$item.quantity"}}}}},{$unset:["purchaseItems.product.code","purchaseItems.product.minimum","purchaseItems.product.cost","purchaseItems.product.srp","purchaseItems.product.startEffectiveDate"]}])