Please add support to delete specialization , while preserving base class instance (multi-table inheritance)

[Piotr Czachur]

Example:
I got multi-table inheritance: Base(Model) and Specific(Base). I got instance of Specific. Sometimes there is a need to delete only Specific leaving Base part in database. Currently if I call delete() on specialization, recursive delete takes place, so Base instance is being deleted too.

[Russell Keith-Magee]

This can be done right now by setting the parent link to None on the child object prior to deletion. This is at least as intuitive as any specific API for this feature that I can think of.

[Piotr Czachur]

It doesn't seem to work.

Please take a look on this sample code:

class List(models.Model):      
    name = models.CharField(max_length=10) 
  
class SpecificList(List):      
    specific_name = models.CharField(max_length=10) 

Let's create specialization instance

>>> s = SpecificList(name='bear', specific_name='sasquatch')
>>> s.save()
110315 11:48:35	   21 Query	INSERT INTO `list_list` (`name`) VALUES ('bear')
		   21 Query	commit
		   21 Query	SELECT ** FROM `list_specificlist` WHERE `list_specificlist`.`list_ptr_id` = 3  LIMIT 1
		   21 Query	INSERT INTO `list_specificlist` (`list_ptr_id`, `specific_name`) VALUES (3, 'sasquatch')
		   21 Query	commit

And now delete (upcast) it as you suggested:

>>> s.list_ptr = None
Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "/srv/django-bugi/nowa/mypro/django/db/models/fields/related.py", line 327, in __set__
    (instance._meta.object_name, self.field.name))
ValueError: Cannot assign None: "SpecificList.list_ptr" does not allow null values.

I also tried setting *_ptr_id to None and then delete:

>>> s.list_ptr_id = None
>>> s.delete()
Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "/srv/django-bugi/nowa/mypro/django/db/models/base.py", line 577, in delete
    assert self._get_pk_val() is not None, "%s object can't be deleted because its %s attribute is set to None." % (self._meta.object_name, self._meta.pk.attname)
AssertionError: SpecificList object can't be deleted because its list_ptr_id attribute is set to None.

This does not work either.

Do I misunderstood you or this feature just doesn't work?

[Julien Phalip]

Please do not reopen a ticket closed by a core dev. If you would like to tackle this issue please bring it up to the django-developers mailing list.

[Russell Keith-Magee]

My apologies -- it appears that your original report was correct. I closed it because I thought I had code in production that was using the trick I suggested; however, when I checked, it turns out I'm reparenting a child, not deleting it.

[Aymeric Augustin]

Change UI/UX from NULL to False.

[Aymeric Augustin]

Change Easy pickings from NULL to False.

[Andriy Sokolovskiy]

​https://github.com/django/django/pull/4074

[Tim Graham <timograham@…>]

In 81c2d9f6:

Fixed #15579 -- Added ability to delete only child models in multi-table inheritance.

[Tim Graham <timograham@…>]

In 71c638fc:

Fixed typo in refs #15579 comment.