2👍
Don’t do it in save(), it’s easily messed up with django. Try to use django signal post_save():
from django.db.models.signals import post_save
@receiver(post_save, sender=Voucher)
def decrease_quota(sender, instance, created, *args, **kwargs):
if created:
instance.vc -= 1
instance.save()
Check django doc for django signals.
0👍
I’d take a slight performance hit in favour of avoiding the duplication of the voucher count information (call me old-fashioned).
In stead of saving the voucher count in Person.vc, I’d evaluate the voucher count when needed by creating a function for that in the Voucher object manager:
def get_voucher_count(self, person):
return self.filter(ps=person).count()
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0👍
OK, I manage to work it out (with the help of Shang Wang & Ytsen de Boer answer as well, which I’m grateful for). The way which I did it was to create an instance of the object Person and take away from it there.
Fortunately enough, I quick time check the doc’s which Shang just showed me (and look way deeper as well). Below is the way to do it:
@receiver(post_save, sender = Voucher, dispatch_uid = "voucher_identifier")
def decrease_quota(sender, instance, **kwargs):
obj = Person.objects.get(pk = instance.ps.id)
if obj.vc < 4 and obj.vc > 0:
obj.vc = obj.vc - 1
obj.save()
Of course I have to do abit more to it (simple validations and stuff like that), but this how I needed it.