How to Update models field based on another field of same models - django

Question

class PurchaseOrder(models.Model):
purchase_order_id = models.AutoField(primary_key=True)
purchase_order_number = models.CharField(unique=True)
vendor = models.ForeignKey(Vendor)
i am creating Purchase Order(po) table. when po created i have to update purchase_order_number as "PO0"+purchase_order_id ex PO0123 (123 is Primary key). so i am using def save in models to accomplish this
def save(self):
if self.purchase_order_id is not None:
self.purchase_order_number = "PO"+str(self.purchase_order_id)
return super(PurchaseOrder, self).save()
It is working fine with single creation but when i try to create bulk of data using locust(Testing tool) its giving an error duplicate entry for PurchseOrdernumber Can we modify field value in models itself some thing like this
purchase_order_number = models.CharField(unique=True,default=("PO"+self.purchase_order_id )

Answer 1

To be honest, I don't think it should work when you create multiple instances. Because as I can see from the code:
if self.purchase_order_id is not None:
self.purchase_order_number = "PO"+str(self.purchase_order_id)
Here purchase_order_id will be None when you are creating new instance. Also, until you call super(PurchaseOrder, self).save(), it will not generate purchase_order_id, meaning purchase_order_number will be empty.
So, what I would recommend is to not store this information in DB. Its basically the same as purchase_order_id with PO in front of it. Instead you can use a property method to get the same value. Like this:
class PurchaseOrder(models.Model):
purchase_order_id = models.AutoField(primary_key=True)
# need to remove `purchase_order_number = models.CharField(unique=True)`
...
#property
def purchase_order_number(self):
return "PO{}".format(self.purchase_order_id)
So, you can also see the purchase_order_number like this:
p = PurchaseOrder.objects.first()
p.purchase_order_number
Downside of this solution is that, you can't make any query on the property field. But I don't think it would be necessary anyway, because you can do the same query for the purchase_order_id, ie PurchaseOrder.objects.filter(purchase_order_id=1).