Deleting child table does not delete rows in parent table when using multi-table inheritance

[Stephen Finucane]

If you have a child model that depends on a parent model via multi-table inheritance, then deleting the child will not delete rows in the parent model's table.

I am aware that we don't use database-level cascades (ticket:21961) but I expected this to be implemented by Django in Python, given the docs (​https://docs.djangoproject.com/en/4.2/topics/db/models/#multi-table-inheritance) say that the automatically-created OneToOneField on the child model will have on_delete=models.CASCADE.

place_ptr = models.OneToOneField(
    Place,
    on_delete=models.CASCADE,
    parent_link=True,
    primary_key=True,
)

I have a minimal reproducer which can be found at ​https://github.com/stephenfin/django-bug-34796. In summary though, you simply need to create a table using multi-table inheritance and then delete that table. While the child table will be deleted, the corresponding rows in the parent table will remain.

[Mariusz Felisiak]

Thanks for the ticket, however migrations doesn't automatically manipulate (delete/update/add) data. Generated migrations are responsible for propagating changes in the database structure (DDL). Any changes to this would be backward incompatible. #21961 is the ticket that must be implemented (as you already noticed) if you want to tell your db that related data should be deleted in cascade.

Marking as a duplicate because #21961 will effectively fix this, for users who opt-in for the cascade behavior.

[Stephen Finucane]

Thanks, Mariusz. However, there is no reference to this anywhere in the documentation, so at a minimum it is very surprising behaviour that is likely to catch users out. Until we fix this in a backwards-incompatible manner, would you be open to a documentation patch that calls this out? Alternatively (better), could we log a warning when auto-generating migrations that remove child tables in a multi-table relationship to indicate this potential issue and suggest users clean it up accordingly?

[Mariusz Felisiak]

Replying to Stephen Finucane:

Thanks, Mariusz. However, there is no reference to this anywhere in the documentation, so at a minimum it is very surprising behaviour that is likely to catch users out.

I don't see anything unexpected in the current behavior. TBH, I would be surprised if migrations manipulated the data in tables on its own.

[Simon Charette]

Also a dupe of #32724.