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([{"$lookup":{"from":"product","let":{purchaseItems:"$purchaseItems"},"pipeline":[{$match:{$expr:{$in:["$_id","$$purchaseItems.product"]}}},{$addFields:{quantity:{$filter:{input:"$$purchaseItems",as:"purch",cond:{$eq:["$$purch.product","$_id"]}}}}},{$unwind:"$quantity"},{$project:{_id:0,product:{_id:"$_id",name:"$name"},quantity:"$quantity.quantity"}}],"as":"purchaseItems"}}])