Deprecate passing unsaved objects to related filters.

[Mapiarz]

Consider this filter:

Foo.objects.filter(related_obj=bar)

Where 'bar' is an unsaved object instance. In Django 1.11, this would always return an empty QuerySet (since no Foo object is related to unsaved 'bar'). In Django 2.0 through 2.2, this is equivalent to doing (which can return a non-empty QuerySet):

Foo.objects.filter(related_obj=None)

I found a somewhat related issue that touches on this subject: https://code.djangoproject.com/ticket/27985

My questions:

1. What is the intended behaviour? In the aforementioned issue Simon Charette suggests that unsaved objects should be prevented from being used in related filters. I agree with that.
2. Is this documented anywhere? I couldn't find anything. At the very least this should be documented somewhere.

[Simon Charette]

Regarding this comment I still think that deprecating passing unsaved objects to related filters is worth doing so I'll accept this ticket on this basis.

Mapiarz, would you be interested in ​submitting a patch doing so? It might require a bit of adjustments in the suite but I think that warning on obj.pk is None is the way to go as it's less controversial than obj._state.adding.

[Hasan Ramezani]

​PR

[Hasan Ramezani]

As I ​commented on PR:

I don't know how to distinguish between p2.choice_set.all() and Choice.objects.filter(poll=p2), It seems get_normalized_value receives the same (value and lhs) for these both cases.

I've also tried to implement it in RelatedLookupMixin.get_prep_lookup based on ​Mariusz comment on PR, but still same problem. the lhs and rhs have the same value in p2.choice_set.all() and Choice.objects.filter(poll=p2) cases.

[Vojtěch Oram]

Could someone please look at this issue? It looks like related merge request is left unfinished. We recently had pretty severe bug because of this. This is really dangerous behavior.

[raydeal]

PR was closed because of (​Mariusz comment on PR) ticket #7488 - admin panel might use filtering with not saved model. But it is 14 years old ticket and it is worth to check, in current version, if admin still use that.

[raydeal]

After investigation of this ticket and #7488 conclusion is:
Issue in the #7488 ticket is obsolete. It is about get_querystet code that haven't existed since 2008.
Time line is as follow:
19-07-2008 newform-admin was merged with main branch
23-07-2008 #7488 ticket was closed
14-11-2008 the BaseInlineFormSet.get_queryset code (forms/models.py) mentioned in #7488 was deleted in #9076 (not closed yet ???) (https://code.djangoproject.com/ticket/9076#comment:22) and queryset moved to

BaseInlineFormSet.__init__

​https://github.com/django/django/commit/bca14cd3c8685f0c8d6a24583e3de33f94f8910b#diff-360e00ebf46ef996c61729321d5a59992d78be2ad6913fb546394c2817b3837a
28-12-2012 this code was changed #19524
20-11-2013 this code was changed #21472
Now the code is

        if self.instance.pk is not None:
            qs = queryset.filter(**{self.fk.name: self.instance})
        else:
            qs = queryset.none()

It means that passing of unsaved object to related filter in admin Inlines is already solved. Ticket #9076 might be closed.

Apart from this, the test ​https://github.com/django/django/blob/4f8c7fd9d91b35e2c2922de4bb50c8c8066cbbc6/tests/model_inheritance_regress/tests.py#L185 could be renamed and used as it was in ​PR

[raydeal]

Replying to Hasan Ramezani:

As I ​commented on PR:

I don't know how to distinguish between p2.choice_set.all() and Choice.objects.filter(poll=p2), It seems get_normalized_value receives the same (value and lhs) for these both cases.

I've also tried to implement it in RelatedLookupMixin.get_prep_lookup based on ​Mariusz comment on PR, but still same problem. the lhs and rhs have the same value in p2.choice_set.all() and Choice.objects.filter(poll=p2) cases.

After looking at code and trying implement yours solution, my conclusion is: it is connected with #19580, if FK raises ValueError like M2M does it will open door to fix this without a struggle how to distinguish between p2.choice_set.all() and Choice.objects.filter(poll=p2) because first case will raise ValueError before filter is evaluated.

[Mariusz Felisiak]

Agreed, test_issue_7488 is obsolete, removed in 18245b948bf7032a0b50d92a743eff822f5bc6a6.

[Mariusz Felisiak <felisiak.mariusz@…>]

In 2b6a3bae:

Fixed #31486 -- Deprecated passing unsaved objects to related filters.

Co-Authored-By: Hasan Ramezani <hasan.r67@…>

[Mariusz Felisiak <felisiak.mariusz@…>]

In 4d78d73:

Refs #31486 -- Removed ability to pass unsaved model instances to related filters.

Per deprecation timeline.