On Tuesday, July 25, 2017 at 5:34:33 AM UTC+3, Nick Stefan wrote:
>
> 5. Q: implicit foreign keys between child to parent in model inheritance?
> A: We could forbid models.DB_CASCADE on these types of relations?
>
There's actually a variation of this that seems a bit hairy to solve
properly. For models:
class Category:
pass
class Child:
pass
class Parent(Child):
category = models.ForeignKey(Category, on_delete=DB_CASCADE)
Then, when you delete a category instance, the associated parent instance
will get deleted, but the child instance won't (as there's no foreign key
from child to parent, we can't make this work in the DB without using
triggers).
Now, the problem here is that this actually leaves the database in an
inconsistent state. I guess the solution might be to document this, add a
warning to checks done on startup, and let users deal with the
inconsistency if they so decide.
We could solve these problems by including an O2O from both parent to child
and from child to parent, and making them both DB_CASCADE. That's not a
good idea for other reasons (complexity of saving a new instance to the
database for one). If we want to make these cases work seamlessly directly
in the database, it's likely better to add in triggers. It wouldn't be
horribly complex code to write. The hardest part would be making sure the
triggers are kept in sync when doing migrations.
For the other parts your proposal does make a lot of sense to me. Don't
fall in the trap of complexities in parent -> child cascade. Just add
warnings and that's it.
- Anssi
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