Hi All, I've taken interest to implimenting built in django support of ON DELETE CASCADE. As a general concept, the ticket was accepted 3 years ago, and there was some discussion around that time: https://code.djangoproject.com/ticket/21961 . I've put together a small PR with some but not all of the proposed design decisions, https://github.com/django/django/pull/8661. The original ticket proposed adding something like models.DB_CASCADE in addition to models.CASCADE, models.SET_NULL. That is the tact that I took in the PR.
At the time of the ticket there were some concerns: 1. Q: What do we do with models.DB_CASCADE when using a DB that doesn't support it? A: MySQL, PostgreSQL, SQLite3 and Oracle all now support this functionality. 2. Q: Won't it be confusing that signals will not fire? A: We can document that the application code mimics models.DO_NOTHING, no signals will fire, and that all the deletion will be handled by SQL. 3. Q: Won't it be confusing that models.DB_CASCADE won't trigger a foreign model's models.CASCADE? A: Is it any different than having a models.DO_NOTHING table pointing to a models.CASCADE table? We can document that on the application side, models.DB_CASCADE works like models.DO_NOTHING. 4. Q: generic foreign keys? A: We could forbid models.DB_CASCADE on these types of relations? 5. Q: implicit foreign keys between child to parent in model inheritance? A: We could forbid models.DB_CASCADE on these types of relations? Additionally there was a proposal by Carl Meyer to split out database based delete into a separate kwarg. To summarize: ForeignKey(on_delete=models.CASCADE) versus ForeignKey(on_delete_db=models.DB_CASCADE). This would help avoid having to implement a magical fallback mode when the DB didn't support models.DB_CASCADE. It also would help eliminate two very different code paths overloading the same kwarg. My thoughts on the above splitting of the kwargs: All of the django 2.0 databases can support ON DELETE CASCADE. So there's no longer any need for a magical fall back mode. There's also the added burden on explaining that we can only pass one of the two kwargs, and not use both at the same time. I think that favors overloading a single kwarg. Simon Charrette made a suggestion to make the models.CASCADE, models.SET_NULL, models.DB_CASCADE, etc all inherit from an OnDelete class. This allows the old application callables to still be simple callables, new SQL based operations such as DB_CASCADE to access an as_sql method, and yet both be instances of the same class for migration serialization. Seeking feedback, Nick -- You received this message because you are subscribed to the Google Groups "Django developers (Contributions to Django itself)" group. To unsubscribe from this group and stop receiving emails from it, send an email to django-developers+unsubscr...@googlegroups.com. To post to this group, send email to django-developers@googlegroups.com. Visit this group at https://groups.google.com/group/django-developers. To view this discussion on the web visit https://groups.google.com/d/msgid/django-developers/20f67b4e-bf5b-4c55-bcc8-aac41cfae598%40googlegroups.com. For more options, visit https://groups.google.com/d/optout.
