#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
 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 = [
                 OpClass(Lower("name"), name="text_pattern_ops"),

 place = Place(name="Narnia")

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

 Full SQL query that is generated:
 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

 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
 oss/django/actions/runs/8171237603/job/22339033423?pr=2 showing the

 === 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:

 === 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
Ticket URL: <https://code.djangoproject.com/ticket/35275>
Django <https://code.djangoproject.com/>
The Web framework for perfectionists with deadlines.

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