Detecting uniqueness doesn't work for models with functional unique constraints.

[Hervé Le Roy]

When creating a new object from Django Administration site, I'm getting an error "Tag with this already exists.". (Tag is my model name)

I'm using on this model the new functional unique constraints introducted in Django 4.0 (​https://docs.djangoproject.com/en/dev/ref/models/constraints/#django.db.models.UniqueConstraint).

I suppose this is related to the contraints put on the model. However, creating a Tag object with code - e.g. Tag.objects.create(...) - works fine. It only fails in Django admin. I'm not sure if it's a bug or misunderstanding/misconfiguration on my side?

This is my model:

class Tag(models.Model):
    """Basic tag model."""
    # When using get_or_create, we need to pass tag name as a default value, like this:
    # Tag.objects.get_or_create(defaults={'name': tag}, name__iexact=tag, user=request.user)
    user = models.ForeignKey(settings.AUTH_USER_MODEL, on_delete=models.CASCADE, verbose_name=_('user'))
    name = models.CharField(max_length=50, db_index=False, verbose_name=_('name'),
                            blank=False,  # It's the default behavior, but let's be explicit
                            validators=[RegexValidator(regex=r'[A-z0-9À-ž\s]+',
                                                       message='This field accepts only letters, digits and space.')])
    slug = models.SlugField(max_length=50, verbose_name=_('slug'))

    class Meta:
        ordering = ['name']
        # Make sure tag name:
        # - are unique per user
        # - are case insensitive (prevent adding "test" if "Test" already exists)
        # - aren't empty
        constraints = [models.UniqueConstraint(fields=['user', 'name'],
                                               name='%(app_label)s_%(class)s_name_unique_per_user'),
                       models.UniqueConstraint(Lower('name'),
                                               name='%(app_label)s_%(class)s_name_case_insensitive'),
                       models.CheckConstraint(check=models.Q(name__length__gt=0),
                                              name='%(app_label)s_%(class)s_name_not_empty')]
(snip)

This is the admin configuration:

@admin.register(Tag)
class TagAdmin(admin.ModelAdmin):
    list_display = ('pk', 'user', 'name', 'slug')
    list_display_links = ['pk']
    fields = ('user', 'name',)
    list_filter = ('user__email',)
    search_fields = ('name',)

[Tim Graham]

See TicketClosingReasons/UseSupportChannels for ways to get help debugging the issue. Feel free to reopen if you confirm Django is at fault.

[Claude Paroz]

Note you should also be able to test outside the admin with tag = Tag(...); tag.full_clean().

[Hervé Le Roy]

Thank you Claude for pointing this out. I was testing outside of admin using tag = Tag(...); tag.save(), which works.

But following your example, when I run tag.full_clean(), it fails with the following error:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python3.9/site-packages/django/db/models/base.py", line 1261, in full_clean
    raise ValidationError(errors)
django.core.exceptions.ValidationError: {'__all__': ['Tag with this  already exists.']}

The fact that there is a missing word between 'this' and 'already' is weird. I have narrowed down the constraint causing the issue (when I remove it, it works fine).
It's this one:

models.UniqueConstraint(Lower('name'), name='%(app_label)s_%(class)s_name_case_insensitive'),

The ability to use positional argument *expressions in UniqueConstraint is a new feature from Django 4.0 ​https://docs.djangoproject.com/en/4.0/ref/models/constraints/#expressions

I'm not expert enough to understand why full_clean()fails. We'll see when 4.0 is released if I'm the only one facing this issue.

[Claude Paroz]

Test code

[Claude Paroz]

I can at least confirm an issue with the error message. As the index has no direct field name, the pattern A <object> with this <field_name> already exists can not work. I attached my code used for testing. In this case, the unique_error_message should probably produce another formulation a bit more generic, like "%(model_name)s is not unique in the database".

I'm not sure if this is release blocker or not.

Hervé, your report suggest an issue also with the unicity check itself, I was not able to reproduce that.

[Mariusz Felisiak]

I was able to confirm the issue with detecting uniqueness. It's a bug in the new feature added in 3aa545281e0c0f9fac93753e3769df9e0334dbaa.

[Mariusz Felisiak]

​Model._get_unique_checks() and related methods don't handle unique constraints with expressions, because _get_unique_checks() returns a constraint without fields which is violated when there are any rows. I can try to solve it on Monday, unless someone does on the weekend.

[Hannes Ljungberg]

PR: ​https://github.com/django/django/pull/15154

Figured we should just exclude UniqueConstraints containing expressions for any logic revolving around ​Options.total_unique_constraints(), we already discard conditional constraints here. I can't imagine any realistic way to run the unique validation checks against expression constraints.

[Mariusz Felisiak]

Replying to Hannes Ljungberg:

PR: ​https://github.com/django/django/pull/15154

Figured we should just exclude UniqueConstraints containing expressions for any logic revolving around ​Options.total_unique_constraints(), we already discard conditional constraints here. I can't imagine any realistic way to run the unique validation checks against expression constraints.

IMO it's feasible to validate against functional constraints, but it would be tricky and it's too late to include this in Django 4.0. expressions could be passed and handled separately in Model._perform_unique_checks() for example by adding custom annotations 🤔 However, let's ignore them for now.

[Mariusz Felisiak <felisiak.mariusz@…>]

In 1eaf38f:

Fixed #33335 -- Made model validation ignore functional unique constraints.

Regression in 3aa545281e0c0f9fac93753e3769df9e0334dbaa.

Thanks Hervé Le Roy for the report.

[Mariusz Felisiak <felisiak.mariusz@…>]

In fed7f992:

[4.0.x] Fixed #33335 -- Made model validation ignore functional unique constraints.

Regression in 3aa545281e0c0f9fac93753e3769df9e0334dbaa.

Thanks Hervé Le Roy for the report.

Backport of 1eaf38fa87384fe26d1abf6e389d6df1600d4d8c from main