UniqueConstraint with condition not validated if the condition field is not editable.

[Márton Salomváry]

Given a model with a UniqueConstraint that has a condition with a field that is not editable
when I try to save this model with an (admin) form with an invalid value for the UniqueConstraint
I expect to get a (Django) validation error.

What I get instead is a database-level integrity error.

If the condition field is editable, I do get a validation error (although a form level one, not a field level one, which might be another bug).
If there is no condition on the field, I do get the expected field level validation error.

Example:

class Thing(models.Model):
    name = models.CharField(max_length=10)

    status = models.CharField(max_length=10, blank=True, editable=False)

    class Meta:
        constraints = [
            models.UniqueConstraint(
                fields=["name"],
                name="thing_unique_name",
                condition=~Q(status="archived"),
            )
        ]

Small test case repo: ​https://github.com/salomvary/django_partial_constraint_bug
(To reproduce, go to ​http://127.0.0.1:8000/admin/thing/thing/add/ and try to add two Things with the same name.)

I've tried the latest dev version of Django, which has same problem.

[Márton Salomváry]

Expected error

[Márton Salomváry]

Actual error

[Márton Salomváry]

The problem seems to be that the non-editable field is passed down as part of excludes, which is right, we do not want to validate the value of the non-editable field.

However this is not about validating the value of the non-editable field, but conditionally validating the value of a *different* field, if the non-editable field's value matches the condition.

The documentation on the semantics of excludes seems to confirm this:
​https://docs.djangoproject.com/en/4.1/ref/models/instances/#django.db.models.Model.full_clean
"ModelForm uses this argument to exclude fields that aren’t present on your form from being validated since any errors raised could not be corrected by the user."

I'm a little confused however because I found a test that seems to specifically verify this (but there is no explanation for the "why" or "for what purpose").
​https://github.com/django/django/blob/e14d08cd894e9d91cb5d9f44ba7532c1a223f458/tests/constraints/tests.py#L698-L703

I've also done a bit of Git archeology and found this conversation around the same problem which ends with "That's a suitable workaround, but I feel like it should not be necessary." ​https://github.com/django/django/pull/14625#pullrequestreview-1074071130

Would a patch be accepted that makes the workaround unnecessary?

[Mariusz Felisiak]

Thanks for the ticket. This is ​the documented behavior for forms and it was ​discussed shortly in the original PR (as you've already noticed).

Unfortunately, changing this would be backward incompatible so please first start a discussion on the DevelopersMailingList, where you'll reach a wider audience and see what other think, and ​follow the guidelines with regards to requesting features.

[Carlton Gibson]

For your project, defining model form subclass for ​ModelAdmin.get_form implementing clean validating the non-editable field would be the way to go.

[Márton Salomváry]

Replying to Mariusz Felisiak:

Thanks for the ticket. This is ​the documented behavior for forms and it was ​discussed shortly in the original PR (as you've already noticed).

Unfortunately, changing this would be backward incompatible so please first start a discussion on the DevelopersMailingList, where you'll reach a wider audience and see what other think, and ​follow the guidelines with regards to requesting features.

I understand, but I'm not sure I have the courage and time to try to sell a breaking change :)

Replying to Carlton Gibson:

For your project, defining model form subclass for ​ModelAdmin.get_form implementing clean validating the non-editable field would be the way to go.

I ended up adding this to the model as a simple workaround (and a handcrafted violation_error_message on the UniqueConstraint):

def clean(self):
    conditional_constraints = {"thing_unique_name"}
    for constraint in self._meta.constraints:
        if constraint.name in conditional_constraints:
            constraint.validate(self.__class__, self)