Misleading ValidationError wording from `limit_choices_to` violation

[Jacob Walls]

The ValidationError raised when a choice violating ForeignKey.limit_choices_to is made refers to an instance "not existing", when in reality, the instance may exist but is simply not an allowed choice.

With the ​example field:

In [5]: Person(staff_member_id=2).full_clean()
---------------------------------------------------------------------------
ValidationError                           Traceback (most recent call last)
Cell In[5], line 1
----> 1 Person(staff_member_id=2).full_clean()

File ~/release/lib/python3.12/site-packages/django/db/models/base.py:1502, in Model.full_clean(self, exclude, validate_unique, validate_constraints)
   1499         errors = e.update_error_dict(errors)
   1501 if errors:
-> 1502     raise ValidationError(errors)

ValidationError: {'staff_member': ['user instance with id 2 does not exist.']}

In [6]: User.objects.get(id=2)
Out[6]: <User: anonymous>

We could use wording like "not a valid choice" and reuse that wording for nonexistent instances as well if there is a disclosure concern about introducing a distinction between the two cases.

[Simon Charette]

Changing the logic to disambiguate between "not exists" and "not matching" would require a non-negligible amount of work (we'd have to annotate the limits_choice_to criteria and check its value instead of simply doing queryset = queryset.complex_filter(...)) and introduces undesirable existence disclosure as you brought up.

Switching the validation error message to "not a valid choice" makes sense though as it prevents disclosure, ​is not an invasive patch, and quite frankly is a better user error message.

[Jacob Walls]

​PR

[Sarah Boyce <42296566+sarahboyce@…>]

In 31837db:

Fixed #35569 -- Improved wording of invalid ForeignKey error message.