My understanding of why you wouldn't want to support using both kwargs at the same time is that it would negate any performance gains from using the DB's delete.
DB_CASCADE has the advantage of not making django run extra queries against every model relation to determine which other tables need cascading delete queries. For example, pretend we've got an app with one particular "shard key", if you will, that relates to about 20 different tables. When I delete that record from the root shard key table, django will run a query against every one of those 20 related tables looking for that key. DB_CASCADE means the DB could just do that for me after deleting one record in the root shard key table with a single query between the server and the database. If we use both, then we have to first run all of those extra queries against the other tables. Its true that they'd be SELECTs versus DELETE, but still queries. Then we'd delete that one record and let the DB do its thing. It's possible that someone would want that, I guess, to allow DB constraints and signals to work together, but it seems rather inefficient. I guess it is possible though. > I'm also curious why you wouldn't put the contraints directly on the DB table instead of injecting them into the SQL commands. In the PR, I actually do use the existing SQL commands that define table columns, but add a little extra constraints (ON DELETE CASCADE). So I believe it is running against the DB table directly? Did I understand you correctly? Nick On Monday, December 11, 2017 at 10:56:31 AM UTC-8, Dylan Young wrote: > > 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. >> > > I'm not clear *why* you wouldn't be able to use both at the same time? > In fact, this seems like the primary benefit of using two separate > kwargs. Can you explain? > > I'm also curious why you wouldn't put the contraints directly on the DB > table instead of injecting them into the SQL commands. > > Can you explain the rationale behind these decisions/constraints? > > > Best, > > Dylan > > On Monday, 24 July 2017 23:34:33 UTC-3, Nick Stefan wrote: >> >> 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 [email protected]. To post to this group, send email to [email protected]. 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/4f6925d6-5ee9-4269-aad9-dc0144f60fd4%40googlegroups.com. 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