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
31475	Recursion issue when deleting related objects	Matt Drew	nobody	"I ran into this situation yesterday... Basically, if you have a cascaded action such as a SET_NULL on a foreign key, you could end up with a RecursionError.

In English: ClassA is a thing.  ClassB has a foreign key to ClassA with on_delete=SET_NULL.  ClassC has a foreign key to ClassB with on_delete=CASCADE. When a ClassA is deleted, a query to get all the ClassB objects that link to ClassA happens in order to update the foreign key to NULL.  This query is optimized to only fetch the id field.

That works fine by itself, but when ClassB has an __init__() that references at least two non-id fields, you get into a loop. When the objects are constructed further queries will be required to fetch the fields referenced in __init__(), but each of those only tries to fetch a single field and instantiate a ClassB with that field, whereupon it finds that it also needs a second ClassB field, so it queries for that field (and ONLY that field), which tries to instantiate a ClassB object with just the second field, which in __init__() requires the first field so it starts another query for the first field, ...

{{{
class ClassA(models.Model):
    some_value = models.CharField(max_length=30)


class ClassB(models.Model):
    foo = models.CharField(max_length=30)
    bar = models.CharField(max_length=30)
    my_a = models.ForeignKey(ClassA, null=True, on_delete=SET_NULL)

    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self._initial_foo = self.foo # Reason for needing this is not shown
        self._initial_bar = self.bar


class ClassC(models.Model):
    baz = models.CharField(max_length=30)
    my_b = models.ForeignKey(ClassB, null=True, on_delete=CASCADE)

a = ClassA(some_value='hi bob')
a.save()
b = ClassB(my_a=a, foo='foo', bar='bar')
b.save()
c = ClassC(my_b=b, baz='baz')
c.save()
ClassA.objects.all().delete()
}}}
Result: RecursionError

If I add a bogus post-delete to ClassB, then django will fetch all the ClassB fields in the query to update the ClassB.my_a references to NULL when a ClassA is deleted, and the recursion doesn't happen.

{{{
@receiver(models.signals.post_delete, sender=ClassB)
def do_nothing(sender, *args, **kwargs):
    pass
}}}

I'm not sure how best to address this.  Maybe some Meta field that lets you specify the minimum fields needed to instantiate an object? Maybe just don't try to only fetch the id column? I'll leave it to you."	Bug	closed	Database layer (models, ORM)	3.0	Normal	invalid			Unreviewed	0	0	0	0	0	0