Django maximum recursion depth exceeded decimal

Question

I'm struggling to try and figure out why a post_save function is returning:

Exception Type:     RuntimeError
Exception Value:    maximum recursion depth exceeded

Here is my code:

#post save functions of the order
def orderPs(sender, instance=False, **kwargs):
    if instance.reference is None:
        instance.reference = str(friendly_id.encode(instance.id))

    #now update the amounts from the order items
    total = 0
    tax = 0
    #oi = OrderItem.objects.filter(order=instance)
    #for i in oi.all():
    #    total += i.total
    #    tax += i.total_tax

    instance.total = total
    instance.tax = tax
    instance.save()


#connect signal
post_save.connect(orderPs, sender=Order)

I've commented out the order items code for now.

instance.total and instance.tax are model decimal fields.

It seems like the post_save function is in an endless loop, not sure why as I've used the same format for all of my post_save functions.

Any ideas?

Maccesch · Accepted Answer · 2012-06-05 02:15:43Z

1

You are calling instance.save() in your post save signal thus triggering it recursively.

All your edited fields in this signal receiver are quite simply derived from other values already stored in the database thus creating redundancies. This is usually not a good idea. Write properties or cached properties instead:

from django.db.models import Sum
from django.utils.functional import cached_property

class Order(model.Model):
    ...
    @cached_property       # or @property
    def total(self):
         return self.orderitem_set.aggregate(total_sum=Sum('total'))['total_sum']

    # do the same with tax

edited Jun 5, 2012 at 2:15

answered Jun 5, 2012 at 1:58

Maccesch

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5 Comments

Ben Kilah Over a year ago

doh, how do I save the new values since the reference is based off the id? Also, the order items aren't created until there is an order object?

Ben Kilah Over a year ago

thanks for the reply.. quick question... if I remove all but the reference generation from my post_save function and call "instance.save()" - is it still going to recursively call?

Maccesch Over a year ago

yes. the instance.save() is the only reason for the recursion and this doesn't depend on whether actual fields have been altered.

Ben Kilah Over a year ago

found this solution fyi: old.marconijr.com/content/…

Maccesch Over a year ago

Thanks! Acutally I thought about that, but in your case that's just fighting the symptoms. You should really try to avoid redundancies in databases!

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