[Django] #34166: UniqueConstraint with condition not validated if the condition field is not editable

[Django]

#34166: UniqueConstraint with condition not validated if the condition field is not
editable
-------------------------------------+-------------------------------------
Reporter: Márton | Owner: nobody
Salomváry |
Type: Bug | Status: new
Component: Database | Version: 4.1
layer (models, ORM) | Keywords: model validation
Severity: Normal | unique constraint condition
Triage Stage: | Has patch: 0
Unreviewed |
Needs documentation: 0 | Needs tests: 0
Patch needs improvement: 0 | Easy pickings: 0
UI/UX: 0 |
-------------------------------------+-------------------------------------
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.

--
Ticket URL: <https://code.djangoproject.com/ticket/34166>
Django <https://code.djangoproject.com/>
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[Django]

#34166: UniqueConstraint with condition not validated if the condition field is not
editable
-------------------------------------+-------------------------------------

Reporter: Márton Salomváry | Owner: nobody
Type: Bug | Status: new
Component: Database layer | Version: 4.1
(models, ORM) |
Severity: Normal | Resolution:
Keywords: model validation | Triage Stage:
unique constraint condition | Unreviewed
Has patch: 0 | Needs documentation: 0

Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------

Changes (by Márton Salomváry):

* Attachment "Screen Shot 2022-11-16 at 16.16.17.png" added.

Expected error

[Django]

#34166: UniqueConstraint with condition not validated if the condition field is not
editable
-------------------------------------+-------------------------------------
Reporter: Márton Salomváry | Owner: nobody
Type: Bug | Status: new
Component: Database layer | Version: 4.1
(models, ORM) |
Severity: Normal | Resolution:
Keywords: model validation | Triage Stage:
unique constraint condition | Unreviewed
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------
Changes (by Márton Salomváry):

* Attachment "Screen Shot 2022-11-16 at 16.18.53.png" added.

Actual error

[Django]

#34166: UniqueConstraint with condition not validated if the condition field is not
editable
-------------------------------------+-------------------------------------
Reporter: Márton Salomváry | Owner: nobody
Type: Bug | Status: new
Component: Database layer | Version: 4.1
(models, ORM) |
Severity: Normal | Resolution:
Keywords: model validation | Triage Stage:
unique constraint condition | Unreviewed
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------

Comment (by 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?

--
Ticket URL: <https://code.djangoproject.com/ticket/34166#comment:1>