[Django] #34007: Single-field conditional UniqueContraint validation errors are clasified as non-field-errors

[Django]

#34007: Single-field conditional UniqueContraint validation errors are 
clasified as
non-field-errors
-----------------------------------------+------------------------
               Reporter:  David Sanders  |          Owner:  nobody
                   Type:  Uncategorized  |         Status:  new
              Component:  Uncategorized  |        Version:  4.1
               Severity:  Normal         |       Keywords:
           Triage Stage:  Unreviewed     |      Has patch:  0
    Needs documentation:  0              |    Needs tests:  0
Patch needs improvement:  0              |  Easy pickings:  0
                  UI/UX:  0              |
-----------------------------------------+------------------------
 Mailing list discussion with Simon Charette: https://groups.google.com/g
 /django-developers/c/7QGy6i9oe4g/m/iCJE7be0AgAJ

 Validating models with UniqueConstraint usually groups errors by field
 name:

 {{{
 class Test(Model):
     test = CharField(max_length=255)

     class Meta:
         constraints = [
             UniqueConstraint(fields=["test"], name="unique"),
         ]

 Test.objects.create(test="abc")
 test = Test(test="abc")
 test.validate_constraints()
 django.core.exceptions.ValidationError: {'test': ['Test with this Test
 already exists.']}
 }}}

 However if a condition is added to the UniqueConstraint the validation
 error is categorised as a non-field error:

 {{{
 class Test(Model):
     test = CharField(max_length=255)

     class Meta:
         constraints = [
             UniqueConstraint(fields=["test"], name="unique",
 condition=~Q(test="")),
         ]

 Test.objects.create(test="abc")
 test = Test(test="abc")
 test.validate_constraints()
 django.core.exceptions.ValidationError: {'__all__': ['Constraint “unique”
 is violated.']}
 }}}

 My proposal is to make the validation error a field-error for any unique
 constraint that only has a single field defined.

 This affects the way custom error messages are defined (eg forms
 error_messages dict vs constraint violation_error_message).

 It's been agreed that the error message wording should remain the generic
 "Constraint is violated" format as determining the correct wording
 involving conditions is non-trivial.

-- 
Ticket URL: <https://code.djangoproject.com/ticket/34007>
Django <https://code.djangoproject.com/>
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