[Django] #35275: Constraints validation fails on UniqueConstraint using OpClass

[Django]

#35275: Constraints validation fails on UniqueConstraint using OpClass
-------------------------------------+-------------------------------------
Reporter: Mathieu | Owner: nobody
Kniewallner |
Type: Bug | Status: new
Component: Database | Version: dev
layer (models, ORM) |
Severity: Normal | Keywords:
Triage Stage: | Has patch: 0
Unreviewed |
Needs documentation: 0 | Needs tests: 0
Patch needs improvement: 0 | Easy pickings: 0
UI/UX: 0 |
-------------------------------------+-------------------------------------
Adding a `UniqueConstraint` using PostgreSQL-specific `OpClass` on a model
breaks constraints validation performed when calling
`validate_constraints` on a model when using PostgreSQL.

=== Minimal reproducer
{{{#!python
from django.contrib.postgres.indexes import OpClass
from django.db import models
from django.db.models.functions import Lower


class Place(models.Model):
name = models.CharField(max_length=255)

class Meta:
app_label = "opclass_issue"
constraints = [
models.UniqueConstraint(
OpClass(Lower("name"), name="text_pattern_ops"),
name="lower_name_uniq",
)
]


place = Place(name="Narnia")
place.validate_constraints()
}}}

This leads to the following error:
{{{
django.db.utils.ProgrammingError: syntax error at or near
"text_pattern_ops"
LINE 1: ...place" WHERE LOWER("opclass_issue_place"."name") text_patte...
}}}

Full SQL query that is generated:
{{{#!sql
SELECT 1 AS "a" FROM "opclass_issue_place" WHERE
LOWER("opclass_issue_place"."name") text_pattern_ops = (LOWER('narnia')
text_pattern_ops) LIMIT 1
}}}

Just in case, this happens even though `django.contrib.postgres` is
correctly installed in `INSTALLED_APPS`, so the issue is not related to
that.

I've also created a test and adapted the CI to run it in
https://github.com/backmarket-oss/django/pull/2/files, which leads to
https://github.com/backmarket-
oss/django/actions/runs/8171237603/job/22339033423?pr=2 showing the
failure.

=== Potential root cause

`OpClass` wrapper should only make sense when creating the constraint, and
should not be used when validating the constraint, as this leads to an
invalid SQL query.

Looking at the code for the PostgreSQL specific `ExclusionConstraint`, a
similar conclusion was reached, and `OpClass` is explicitly removed in
`validate` method:
https://github.com/django/django/blob/4426b1a72dc289643e2ae8c190b8dc4b3a39daf7/django/contrib/postgres/constraints.py#L211-L216.

=== Potential fix

Applying a similar fix as the one above in `UniqueConstraint`, showcased
in https://github.com/backmarket-oss/django/pull/1/files applied on top of
the other PR above, leads to https://github.com/backmarket-
oss/django/actions/runs/8171242801/job/22339050847?pr=1 where the added
test is now passing.

If you think that this is the correct fix to apply, or have a lead for
another fix, I'd be happy to make a proper pull request targeting Django
repository.
--
Ticket URL: <https://code.djangoproject.com/ticket/35275>
Django <https://code.djangoproject.com/>
The Web framework for perfectionists with deadlines.

[Django]

#35275: Constraints validation fails on UniqueConstraint using OpClass
-------------------------------------+-------------------------------------

Reporter: Mathieu Kniewallner | Owner: nobody
Type: Bug | Status: new
Component: Database layer | Version: dev
(models, ORM) |
Severity: Normal | Resolution:
Keywords: | Triage Stage: Accepted
Has patch: 0 | Needs documentation: 0

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

Changes (by Mariusz Felisiak):

* cc: Gagaro (added)
* stage: Unreviewed => Accepted

Comment:

Good catch, thanks for the report. Please prepare a patch via GitHub PR (a
regression test is required).
--
Ticket URL: <https://code.djangoproject.com/ticket/35275#comment:1>

[Django]

#35275: Constraints validation fails on UniqueConstraint using OpClass
-------------------------------------+-------------------------------------

Reporter: Mathieu Kniewallner | Owner: Mathieu
| Kniewallner
Type: Bug | Status: assigned

Component: Database layer | Version: dev
(models, ORM) |
Severity: Normal | Resolution:
Keywords: | Triage Stage: Accepted

Has patch: 1 | Needs documentation: 0

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

Changes (by Mathieu Kniewallner):

* has_patch: 0 => 1
* owner: nobody => Mathieu Kniewallner
* status: new => assigned

--
Ticket URL: <https://code.djangoproject.com/ticket/35275#comment:2>

[Django]

#35275: Constraints validation fails on UniqueConstraint using OpClass
-------------------------------------+-------------------------------------
Reporter: Mathieu Kniewallner | Owner: Mathieu
| Kniewallner
Type: Bug | Status: assigned
Component: Database layer | Version: dev
(models, ORM) |
Severity: Normal | Resolution:
Keywords: | Triage Stage: Accepted
Has patch: 1 | Needs documentation: 0

Needs tests: 0 | Patch needs improvement: 1

Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------

Changes (by Mariusz Felisiak):

* needs_better_patch: 0 => 1

--
Ticket URL: <https://code.djangoproject.com/ticket/35275#comment:3>

[Django]

#35275: Constraints validation fails on UniqueConstraint using OpClass
-------------------------------------+-------------------------------------

Reporter: Mathieu Kniewallner | Owner: Mariusz
| Felisiak

Type: Bug | Status: assigned
Component: Database layer | Version: dev
(models, ORM) |
Severity: Normal | Resolution:
Keywords: | Triage Stage: Accepted
Has patch: 1 | Needs documentation: 0

Needs tests: 0 | Patch needs improvement: 0

Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------
Changes (by Mariusz Felisiak):

* needs_better_patch: 1 => 0
* owner: Mathieu Kniewallner => Mariusz Felisiak

Comment:

[https://github.com/django/django/pull/18160 PR]
--
Ticket URL: <https://code.djangoproject.com/ticket/35275#comment:4>

[Django]

#35275: Constraints validation fails on UniqueConstraint using OpClass
-------------------------------------+-------------------------------------
Reporter: Mathieu Kniewallner | Owner: Mariusz
| Felisiak
Type: Bug | Status: assigned
Component: Database layer | Version: dev
(models, ORM) |
Severity: Normal | Resolution:

Keywords: | Triage Stage: Ready for
| checkin

Has patch: 1 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------

Changes (by Sarah Boyce):

* stage: Accepted => Ready for checkin

--
Ticket URL: <https://code.djangoproject.com/ticket/35275#comment:5>

[Django]

#35275: Constraints validation fails on UniqueConstraint using OpClass
-------------------------------------+-------------------------------------
Reporter: Mathieu Kniewallner | Owner: Mariusz
| Felisiak

Type: Bug | Status: closed

Component: Database layer | Version: dev
(models, ORM) |

Severity: Normal | Resolution: fixed

Keywords: | Triage Stage: Ready for
| checkin
Has patch: 1 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------

Changes (by Sarah Boyce <42296566+sarahboyce@…>):

* resolution: => fixed
* status: assigned => closed

Comment:

In [changeset:"f030236a86a64a4befd3cc8093e2bbeceef52a31" f030236a]:
{{{#!CommitTicketReference repository=""
revision="f030236a86a64a4befd3cc8093e2bbeceef52a31"
Fixed #35275 -- Fixed Meta.constraints validation crash on
UniqueConstraint with OpClass().

This also introduces Expression.constraint_validation_compatible that
allows specifying that expression should be ignored during a constraint
validation.
}}}
--
Ticket URL: <https://code.djangoproject.com/ticket/35275#comment:6>