Single-field conditional UniqueContraint validation errors are classified as non-field-errors

[David Sanders]

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.

[David Sanders]

PR: ​https://github.com/django/django/pull/16054

[Sarah Boyce <42296566+sarahboyce@…>]

In 97d48cd:

Refs #34007, Refs #35359 -- Added Q.referenced_based_fields property.

Thank you to Mariusz Felisiak and Natalia Bidart for the reviews.

[Natalia <124304+nessita@…>]

In fa202d5c:

[5.0.x] Refs #34007, Refs #35359 -- Added Q.referenced_based_fields property.

Thank you to Mariusz Felisiak and Natalia Bidart for the reviews.

Backport of 97d48cd3c6f409584b5cc19fbddfca917bae78fd from main