on_delete=models.SET_NULL should be the default for nullable FKs
|Reported by:||carljm||Owned by:||nobody|
|Component:||Database layer (models, ORM)||Version:||1.5|
|Has patch:||no||Needs documentation:||no|
|Needs tests:||no||Patch needs improvement:||no|
This wasn't done when we added the on_delete feature due to concerns about breaking backwards-compatibility with the previous always-cascade-deletes behavior. But if you set aside legacy considerations, it seems intuitively obvious (to me anyway) that SET_NULL is a superior default for a nullable FK than CASCADE. And even if others might have a different intuition, the consequences of your intuition being wrong are far from symmetrical: one way you get a model instance with a null FK hanging around that you didn't expect to still have, the other way you get data loss.
The trick, of course, is that changing default values is hard to do with a deprecation process. For a deprecation warning to catch the people it needs to catch, you have to warn anytime an explicit value is not specified. So you have to force everybody to stop relying on the default in order to silence the deprecation warnings, which reduces the value of having a new default, in the short run anyway.
I think it's still worth doing for the long run, though.
We could also consider doing it without a deprecation process, just a backwards-incompatible note in the release notes. Since the consequences are the opposite of data loss, that might be ok. Thoughts?