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
18306	Deferred models should automatically issue update_fields when saving	Anssi Kääriäinen	Anssi Kääriäinen	"When saving a model fetched through deferred model loading (.only() or .defer()) and then saving these models it would be useful to issue an update_fields automatically.

Currently, all the model's deferred fields are fetched from the DB and then saved back - this doesn't make sense. Instead, doing
{{{
(assume Author with fields age + name)
a = Author.objects.only('name').get(pk=1)
a.name = 'Anssi'
a.save()
}}}
should be equivalent to doing:
{{{
a = Author.objects.only('name').get(pk=1)
a.name = 'Anssi'
a.save(update_fields=['name'])
}}}

In addition, the following must work:
{{{
a = Author.objects.only('name').get(pk=1)
a.name = 'Anssi'
a.age = 31
a.save()
}}}
both age and name must be saved in this case.

Also, if save is given update_fields argument, it must override the deferred model argument.

I would like to investigate the possibility of storing the fields to update in model._state.update_fields - the reason is that this would be a nice hook for 3rd party model subclasses which could use that variable to implement automatic tracking of changed fields. There is a lot of discussion of this feature in #4102, and the decision there seems clear: we can't include automatic change tracking in Django core. However, allowing easy creation of state-tracking subclasses would be a nice addition."	Cleanup/optimization	closed	Database layer (models, ORM)	dev	Normal	fixed		niwi@…	Ready for checkin	1	0	0	0	0	0