id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
20272	Model._do_update isn't as useful as it could be	gavinwahl@…	nobody	"Ticket #16649 refactored model saving and added a `_do_update` method that does the actual database UPDATE query. However, a similar database query that checks that a row with the pk still exists under certain conditions is still in `_save_base`, and as such can't be overridden by a subclass.

`_save_base` shortcuts `_do_update` when (from the comment):

  We can end up here when saving a model in inheritance chain where update_fields doesn't target any field in current model. In that case we just say the update succeeded. Another case ending up here is a model with just PK - in that case check that the PK still exists.

I am using `_do_update` to implement optimistic concurrency control in [[https://github.com/gavinwahl/django-optimistic-lock/|django-optimistic-lock]].  In order to implement it correctly for inherited models, I need to add my own condition to the EXISTS query that `_save_base` does. If this query were pushed down to `_do_update`, I would be able to do this. Otherwise, I can't add my logic because it is deep inside the `_save_base` code.

I have a failing test in my library that demonstrates: https://github.com/gavinwahl/django-optimistic-lock/blob/4472b73b3eeb7ce2ea19164c75d70aa39249aa34/tests/tests/tests.py#L72

I modified Django slightly to move the existence query into `_do_update`, and I'm able to implement the behavior I need for my library. See https://github.com/gavinwahl/django-optimistic-lock/commit/90be77cb356a7b65410df3f52407cc38da2f8e22.

My patch: https://github.com/gavinwahl/django/commit/cfebca2f11655b64b21cab554371c2696d4faa36/
"	Cleanup/optimization	closed	Database layer (models, ORM)	dev	Normal	fixed			Ready for checkin	1	0	0	0	0	0