Unique together constraints including the locale cause IntregrityError

Issue Summary

When using the locale in unique_together or a UniqueConstraint on a snippet then submitting a violating form leads to a IntegrityError.

Steps to Reproduce

1. Get a current version of the Wagtail bakery demo project.

2. Edit bakerydemo/breads/models.py as follows.

   @register_snippet
   class BreadCrumbs(TranslatableMixin, models.Model):
       title = models.CharField(max_length=255)
   
       panels = [
           FieldPanel('title'),
       ]
   
       def __str__(self):
           return self.title
   
       class Meta(TranslatableMixin.Meta):
           unique_together = TranslatableMixin.Meta.unique_together + [
               ('title', 'locale'),
           ]
   

3. Run ./manage.py makemigrations && ./manage.py migrate && ./manage.py runserver.

4. Go to http://localhost:8000/admin/snippets/breads/breadcrumbs/

5. Click the “add bread cumbs” button

6. Enter “test” in the title field and save the snippet.

7. Click the “add bread cumbs” button again

8. Enter “test” in the title field and save the snippet agin.

9. Observe the IntegrityError:

Any other relevant information. For example, why do you consider this a bug and what did you expect to happen instead?

The expected behavior would be to see an error message like the following:

I have tried other variants of unique_together constrains, e.g. two char fields or a char field and another foreign key field (other than the locale) and they all work and lead to nice error messages like the one shown above.

Here is a different example snippet with a working unique_together constraint to a foreign key.

@register_snippet
class CrumbType(models.Model):
    title = models.CharField(max_length=255)
    ingredient = models.ForeignKey(to=BreadIngredient, related_name="+", on_delete=models.PROTECT)

    panels = [
        FieldPanel('title'),
        FieldPanel('ingredient'),
    ]

    def __str__(self):
        return self.title

    class Meta:
        unique_together = [
            ('title', 'ingredient'),
        ]

• I have confirmed that this issue can be reproduced as described on a fresh Wagtail project: (yes / no)

Technical details

• Python version: Run python --version. Python 3.9.13
• Django version: Look in your requirements.txt, or run pip show django | grep Version. Version: 4.0.5
• Wagtail version: Look at the bottom of the Settings menu in the Wagtail admin, or run pip show wagtail | grep Version:. Version: 3.0.1
• Browser version: You can use https://www.whatsmybrowser.org/ to find this out. – Not a browser issue -> Any

[allcaps]

The ‘locale’ field has ‘editable=False’ iirc.

Field.editable¶ If False, the field will not be displayed in the admin or any other ModelForm. They are also skipped during model validation. Default is True. https://docs.djangoproject.com/en/4.1/ref/models/fields/#editable

I do get why direct editing of this field is prohibited, but I wouldn’t be against a less restrictive approach and make this field editable by removing the attr.

Same situation applies for the uid field.

[tbrlpld]

Hi @Sanyam-Garg. Thanks for replicating this!

Unfortunately I am unfamiliar with the view / validation logic myself, so I won’t be of much help there.