Another cascading bug on CASCADE deletion

[godardth]

Hi,

I have an issue in django-polymorphic when deleting models from django-admin.

Models are defined as below

class Farm(models.Model):
    class Meta:
        pass

class Animal(PolymorphicModel):
    farm = models.ForeignKey('Farm', on_delete=models.CASCADE)
    name = models.CharField(max_length=100)
    class Meta:
        pass

class Dog(Animal):
    dog_param = models.CharField(max_length=100)
    class Meta:
        pass

class Cat(Animal):
    cat_param = models.CharField(max_length=100)
    class Meta:
        pass

Now I am creating a Farm object with both a Dog and a Cat (problem doesn't appear if all polymorphic related objects are of the same sub-class.

When trying to delete the Farm object in admin, I get the below hierarchy

• Farm: Farm number 19
  • Dog: 37 - dog
  • Cat: 38 - cat
• Animal: 37 - dog
• Animal: 38 - cat

And when confirming the deletion, I get the error below

Internal Server Error: /admin/farming/farm/
Traceback (most recent call last):
  File "/usr/local/lib/python2.7/dist-packages/django/core/handlers/base.py", line 149, in get_response
    response = self.process_exception_by_middleware(e, request)
  File "/usr/local/lib/python2.7/dist-packages/django/core/handlers/base.py", line 147, in get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "/usr/local/lib/python2.7/dist-packages/django/contrib/admin/options.py", line 541, in wrapper
    return self.admin_site.admin_view(view)(*args, **kwargs)
  File "/usr/local/lib/python2.7/dist-packages/django/utils/decorators.py", line 149, in _wrapped_view
    response = view_func(request, *args, **kwargs)
  File "/usr/local/lib/python2.7/dist-packages/django/views/decorators/cache.py", line 57, in _wrapped_view_func
    response = view_func(request, *args, **kwargs)
  File "/usr/local/lib/python2.7/dist-packages/django/contrib/admin/sites.py", line 244, in inner
    return view(request, *args, **kwargs)
  File "/usr/local/lib/python2.7/dist-packages/django/utils/decorators.py", line 67, in _wrapper
    return bound_func(*args, **kwargs)
  File "/usr/local/lib/python2.7/dist-packages/django/utils/decorators.py", line 149, in _wrapped_view
    response = view_func(request, *args, **kwargs)
  File "/usr/local/lib/python2.7/dist-packages/django/utils/decorators.py", line 63, in bound_func
    return func.__get__(self, type(self))(*args2, **kwargs2)
  File "/usr/local/lib/python2.7/dist-packages/django/contrib/admin/options.py", line 1512, in changelist_view
    response = self.response_action(request, queryset=cl.get_queryset(request))
  File "/usr/local/lib/python2.7/dist-packages/django/contrib/admin/options.py", line 1245, in response_action
    response = func(self, request, queryset)
  File "/usr/local/lib/python2.7/dist-packages/django/contrib/admin/actions.py", line 49, in delete_selected
    queryset.delete()
  File "/usr/local/lib/python2.7/dist-packages/django/db/models/query.py", line 600, in delete
    deleted, _rows_count = collector.delete()
  File "/usr/local/lib/python2.7/dist-packages/django/db/models/deletion.py", line 308, in delete
    for model, instances in six.iteritems(self.data):
  File "/usr/local/lib/python2.7/dist-packages/django/db/transaction.py", line 223, in __exit__
    connection.commit()
  File "/usr/local/lib/python2.7/dist-packages/django/db/backends/base/base.py", line 242, in commit
    self._commit()
  File "/usr/local/lib/python2.7/dist-packages/django/db/backends/base/base.py", line 211, in _commit
    return self.connection.commit()
  File "/usr/local/lib/python2.7/dist-packages/django/db/utils.py", line 95, in __exit__
    six.reraise(dj_exc_type, dj_exc_value, traceback)
  File "/usr/local/lib/python2.7/dist-packages/django/db/backends/base/base.py", line 211, in _commit
    return self.connection.commit()
IntegrityError: update or delete on table "farming_animal" violates foreign key constraint "farming_cat_animal_ptr_id_77dae4b8_fk_farming_animal_id" on table "farming_cat"
DETAIL:  Key (id)=(38) is still referenced from table "farming_cat".

I tracked the problem down to be a problem in the deletion Collector class where related object to Farm (i.e. Animal) are of the sub-classes either Dog or Cat. So in the case above, we try to delete the Cat object from the Dog table.

I solved my problem by using the _base_manager property of the Model class . And referring to a non polymorphic object manager like below

class Animal(PolymorphicModel):
    instance = models.ForeignKey('Farm', on_delete=models.CASCADE)
    name = models.CharField(max_length=100)
    _base_manager = models.Manager()
    class Meta:
        pass

Now when trying to delete the Farm object in admin I get the correct hierarchy

• Farm: Farm number 19
  • Animal: Fix model subclass ___ selector for abstract/proxy models #37 - dog
    • Dog: Fix model subclass ___ selector for abstract/proxy models #37 - dog
  • Animal: Simplify add_type_form.html #38 - cat
    • Cat: Simplify add_type_form.html #38 - cat

I am using the below

psql 9.3.11
python 2.7.6
django 1.9.8
django-polymorphic 0.9.2

My thought would be to add _base_manager = models.Manager() in the django-polymorphic PolymorphicModel. What do you guy think about this ?

Thanks,
Theo

[patient]

Have the problem too, but in Django 1.10 solution with _base_manager does not work. Now it works with:

class Animal(PolymorphicModel):
    ...
    class Meta:
        base_manager_name = 'base_objects'

[x603]

I've faced the same issue when trying to delete objects. Adding base manager solved it. What does it actually do, can somebody explain please? And why without base manager it throws an error? Thanks.

[c0d3z3r0]

... same issue here with Django 1.10. Adding the base_manager_name solved the problem, too.
This seems to be related to #84

[gunnar2k]

Thank you! @patient

[jstray]

This is a horrifying bug -- and took up about two days of my team's effort to understand before we found this workaround.

These seem to be the same bug:
#34
https://code.djangoproject.com/ticket/23076#comment:16

The latter has lots of intricate detail on what is going on. It's complicated. I'm not sure if the correct solution would be to change django or django-polymorphic.

Also, it is likely to be difficult to write a failing test for this bug as sqlite does not enforce constraints, so it only fails on Postgres (and therefore only on a production server, not a development environment!)

Nonetheless it would be really great to fix this.

[meshy]

It's worth noting that tests are now running against postgres.

[benrudolph]

When I updated from 2.0 to 2.0.1 the workaround with base_manager_name no longer works. Had to bump the version back down to 2.0

[mumumumu]

I'm on django 1.11 and django-polymorphic 2.0.2. This is the workaround I'm currently using:

class ABC(PolymorphicModel):

    non_polymorphic = models.Manager()

    class Meta
        base_manager_name = 'non_polymorphic'

[sephii]

I'm getting the same issue as well (Django 1.11, django-polymorphic 2.0.2) and used the fix proposed in #229 (comment). I don't know if this has any other implication so it would be nice to have a proper fix for that.

[chrismeyersfsu]

I worked around by setting my own SET_NULL function that is essentially a wrapper to call sub_objs.non_polymorphic() on Django 1.11.x

from django.db import models

def SET_NULL(collector, field, sub_objs, using):
    return models.SET_NULL(collector, field, sub_objs.non_polymorphic(), using)

    books = models.ForeignKey(
        'Book',
        blank=True,
        null=True,
        default=None,
        on_delete=SET_NULL,
    )