Deletion of objects with a GenericRelation(X) deletes unrelated X objects with the same object_id!

[Thomas Steinacher <tom@…>]

Take a look at the following models file:

from django.db import models
from django.contrib.contenttypes.models import ContentType

class Pointer(models.Model):
    object = models.GenericForeignKey()
    object_id = models.IntegerField()
    content_type = models.ForeignKey(ContentType)

class A(models.Model):
    pointers = models.GenericRelation(Pointer)

class B(models.Model):
    pointers = models.GenericRelation(Pointer)

Let's create an A, a B and two Pointer objects:

In [1]: from test import models
In [2]: a = models.A.objects.create()
In [3]: b = models.B.objects.create()
In [4]: pa = models.Pointer.objects.create(object=a)
In [5]: pb = models.Pointer.objects.create(object=b)
In [6]: [(p.content_type, p.object_id) for p in models.Pointer.objects.all()]
Out[6]: [(<ContentType: a>, 1), (<ContentType: b>, 1)]

Now let's delete ONE object. Notice that ALL pointer objects with this object_id will be deleted. Django DOES NOT take a look at the content_type!

In [7]: a.delete()
In [8]: [(p.content_type, p.object_id) for p in models.Pointer.objects.all()]
Out[8]: []

It looks like the deletion happens in django/db/models/query.py (lines 935-940):

        for f in cls._meta.many_to_many:
            for offset in range(0, len(pk_list), GET_ITERATOR_CHUNK_SIZE):
                cursor.execute("DELETE FROM %s WHERE %s IN (%s)" % \
                    (qn(f.m2m_db_table()), qn(f.m2m_column_name()),
                    ','.join(['%s' for pk in pk_list[offset:offset+GET_ITERATOR_CHUNK_SIZE]])),
                    pk_list[offset:offset+GET_ITERATOR_CHUNK_SIZE])

The pointer class is in the object's many_to_many list, because I added "pointers = models.GenericRelation(Pointer)" to the model:

In [9]: models.A._meta.many_to_many[0].m2m_db_table()
Out[9]: 'test_pointer'
In [10]: models.A._meta.many_to_many[0].m2m_column_name()
Out[10]: 'object_id'

I'm using Django SVN (rev 4269).

[Thomas Steinacher <tom@…>]

Looks like this bug is related to #2749. This should be fixed as soon as possible as it deletes unrelated objects...

[Alex Dedul]

I've already tried to implement fix for this in #3081, but it needs improvements - for now it couples contrib content types to django database wrapper, uses generator expressions available only in python 2.4, lacks tests etc. And if generic relations will be moved to contrib apps too seems like it would be problematic to do proper deletion at all.. But this should be investigated further.

[Russell Keith-Magee]

Test case for deletion problem

[Russell Keith-Magee]

Full patch (including tests) for deleting generic related objects

[Russell Keith-Magee]

Patch needs a sanity check from a pair of eyes that aren't jetlagged.

[Russell Keith-Magee]

(In [4428]) Fixed #3215, #3081, #2749 -- Fixed problem with mistaken deletion of objects when a GenericRelation? is involved. Thanks to Thomas Steinacher for helping to narrow down the problem (#3215), and Alex Dedul (#3081) for the starting point of a working patch.