id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
24576	Bad cascading leads to non-deterministic IntegrityError	Marc Aymerich	nobody	"I've spent all day with a bug on my application that seems to be a bug on Django's ORM delete on cascade resolution order.

Moreover because the order in which related objects are deleted by the ORM is non-deterministic (changes every time you start the interpreter) it makes even harder to track down. 

So far I've been able to reproduce the problem with a few models:


{{{
from django.db import models
from django.contrib.contenttypes.fields import GenericForeignKey, GenericRelation
from django.contrib.contenttypes.models import ContentType
from django.db.models.signals import post_delete
from django.dispatch import receiver

class Resource(models.Model):
    content_type = models.ForeignKey(ContentType)
    object_id = models.PositiveIntegerField()
    content_object = GenericForeignKey()

class Account(models.Model):
    name = models.CharField(max_length=10)
    resources = GenericRelation(Resource)

class Order(models.Model):
    account = models.ForeignKey(Account)
    content_type = models.ForeignKey(ContentType)
    object_id = models.PositiveIntegerField()
    content_object = GenericForeignKey()

class MetricStorage(models.Model):
    order = models.ForeignKey(Order)

@receiver(post_delete, dispatch_uid=""orders.cancel_orders"")
def cancel_orders(sender, **kwargs):
    instance = kwargs['instance']
    print('delete', sender, instance, instance.pk)
    if sender == Resource:
        related = instance.content_object
        if related:
            ct = ContentType.objects.get_for_model(related)
            order = Order.objects.get(content_type=ct, object_id=related.pk)
            order.metricstorage_set.create()
            order.save()
        else:
            print('related is None')
}}}


And this test code


{{{
from test.models import Account, Resource, Order

account = Account.objects.create(name='John')
resource = Resource.objects.create(content_object=account)
order = Order.objects.create(account=account, content_object=account)
account.delete()
}}}

In order to properly test this you should make tries restarting the interpreter a handful of times, and then you'll see two different results.

This that I consider correct:

{{{
delete <class 'test.models.Order'> Order object 384
delete <class 'test.models.Account'> Account object 124
delete <class 'test.models.Resource'> Resource object 307
related is None
}}}

And the IntegrityError which I consider to be a bug


{{{
delete <class 'test.models.Resource'> Resource object 311
delete <class 'test.models.Order'> Order object 388
delete <class 'test.models.Account'> Account object 128
Traceback (most recent call last):
  File ""<console>"", line 1, in <module>
  File ""/usr/local/lib/python3.4/dist-packages/django/db/models/base.py"", line 872, in delete
    collector.delete()
  File ""/usr/local/lib/python3.4/dist-packages/django/db/models/deletion.py"", line 314, in delete
    sender=model, instance=obj, using=self.using
  File ""/usr/local/lib/python3.4/dist-packages/django/db/transaction.py"", line 232, in __exit__
    connection.commit()
  File ""/usr/local/lib/python3.4/dist-packages/django/db/backends/base/base.py"", line 173, in commit
    self._commit()
  File ""/usr/local/lib/python3.4/dist-packages/django/db/backends/base/base.py"", line 142, in _commit
    return self.connection.commit()
  File ""/usr/local/lib/python3.4/dist-packages/django/db/utils.py"", line 97, in __exit__
    six.reraise(dj_exc_type, dj_exc_value, traceback)
  File ""/usr/local/lib/python3.4/dist-packages/django/utils/six.py"", line 658, in reraise
    raise value.with_traceback(tb)
  File ""/usr/local/lib/python3.4/dist-packages/django/db/backends/base/base.py"", line 142, in _commit
    return self.connection.commit()
django.db.utils.IntegrityError: insert or update on table ""test_metricstorage"" violates foreign key constraint ""test_metricstorage_order_id_730c757c66b8c627_fk_test_order_id""
DETAIL:  Key (order_id)=(388) is not present in table ""test_order"".
}}}


Notice how the order in which related objects are deleted is different, I believe this to be the source of the problem.

I've noticed this problem on 1.7 and now 1.8, not tested with master."	Bug	closed	Database layer (models, ORM)	1.8	Normal	fixed		feierlaura10@…	Ready for checkin	1	0	0	0	0	0