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
36668	ModelForm validation skips UniqueConstraint if its condition refers to a field not present on the form	Baptiste Mispelon		",,(sorry for this mouthful of a title 😁),,

For starters, I'm not sure if this is a bug or a feature request but it feels like a bug to me so I went with that. Similarly, I'm unsure if this is a `ModelForm` issue or a `UniqueConstraint` one and went with the latter.

Now for the actual issue, consider this code (see attached patch for an actual testcase [1] ):

{{{#!python
from django import forms
from django.db import models


class Page(models.Model):
    pass


class Revision(models.Model):
    page = models.ForeignKey(Page, on_delete=models.CASCADE)
    status = models.IntegerField(default=1)

    class Meta:
        constraints = [
            models.UniqueConstraint(
                name=""unique_page_status_1"",
                fields=[""page""],
                condition=models.Q(status=1),
            )
        ]


class RevisionForm(forms.ModelForm):
    class Meta:
        model = Revision
        fields = [""page""]


page = Page.objects.create()
page.revision_set.create()

form = RevisionForm(data={""page"": page.pk})
# I would explain the form to be invalid because the revision that would be
# created by saving the form would fail against the constraint.
# But that's not what happens:
assert not form.is_valid()  # fails
}}}
(I've tested as far back as 4.1 and they all exhibit the same issue.)

I've investigated a little and here's what I think is happening: the `ModelForm` ends up calling `UniqueConstraint.validate()` and passing it an `exclude` list that contains `status` (because `status` is a field on the model, but not on the form). In turn, the logic in `UniqueConstraint.validate()` notices that `status` is present in its `condition`, and so decides to skip that validation.

I think that last step where `UniqueConstraint.validate()` skips fields used in the constraint's `condition` if they're listed in `exclude` is a bug. I think this behavior makes sense for `CheckConstraint`, but not for `UniqueConstraint`. In my mind `UniqueConstraint.validate()` should only check `self.fields` and ignore `self.condition`.



[1] Once the patch is applied with `git apply ticket-xxx.diff` you can run it with `uv run --with-editable=. --with-requirements=tests/requirements/py3.txt python tests/runtests.py aaaaaa`"	Bug	closed	Database layer (models, ORM)	5.2	Normal	invalid	modelform,constraint,uniqueconstraint		Unreviewed	0	0	0	0	0	0