Django: unique_together for foreign field

Question

For next two models:

class Foo(models.Model):
    parent = models.ForeignKey(Parent)
    name = models.CharField()


class Bar(models.Model):
    foreign = models.ForeignKey(Foo)
    value = models.CharField(max_length=20)

I need to have unique_together constraint for Bar model:

class Meta:
    unique_together = ('value', 'foreign__parent')

Which is impossible in Django.

But is it possible to achieve this on database level (Postgresql) with some contraint or model level validation to omit possible case (lock table?) when simultaneously same value may be saved to different Bar instances?

Django 2.2.4

https://stackoverflow.com/questions/4440010/django-unique-together-with-foreign-keys — Vaibhav Vishal, Aug 29 '19 at 06:08

score 1 · Answer 1 · answered Aug 29 '19 at 06:08

1

You cannot achieve it with unique_together because it creates an index in the database level. But, You can add validation for it yourself though, simply overwrite the validate_unique method and add this validation to it.

from django.core.exceptions import ValidationError

class Bar(models.Model):
    foreign = models.ForeignKey(Foo)
    value = models.CharField(max_length=20)

    def validate_unique(self, *args, **kwargs):
        super(MyModel, self).validate_unique(*args, **kwargs)

        if self.__class__.objects.\
                filter(foreign__parent=self.foreign.parent, vaue=self.value).exists():
            raise ValidationError(
                message='record already exists with given values.',
                code='unique_together',
            )

answered Aug 29 '19 at 06:08

anjaneyulubatta505

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@Sibirsky I guess it should call `validate_unique` when `is_valid()` method invoked on `serializer`. – anjaneyulubatta505 Aug 29 '19 at 06:29
feels like it doesn't. – Sibirsky Aug 29 '19 at 06:52
did you test it ? – anjaneyulubatta505 Aug 29 '19 at 07:05
Yes, got bunch of unit tests. I can add appropriate validations to serializer, but what about model level? I mean, if I just add `validate_unique()` to `Bar`, it won't trigger by itself if I try to save model - I would need to add this inside of `save()` at least, but that's also not good since I got lots of validations on serializer itself. – Sibirsky Aug 29 '19 at 07:24
It should also work with it. did you try checking it with model `save` method ? – anjaneyulubatta505 Aug 29 '19 at 07:30
Yes. `Model.save()` doesn't trigger `validate_unique()`. – Sibirsky Aug 29 '19 at 07:50
You can override save method and use `Model.full_clean` method inside it. Ref: https://docs.djangoproject.com/en/2.2/_modules/django/db/models/base/#Model.full_clean – anjaneyulubatta505 Aug 29 '19 at 08:00

score 0 · Accepted Answer · answered Aug 29 '19 at 09:19

Thanks to Anjaneyulu Batta, came to next solution:

@contextmanager
def lock_table(model):
    """
    Lock target table on commands UPDATE, DELETE and INSERT
    """
    with transaction.atomic(), transaction.get_connection().cursor() as cursor:
        cursor.execute(
            f'LOCK TABLE {model._meta.db_table} IN ROW EXCLUSIVE MODE;'
        )
        yield

And for model:

def validate_unique(self, exclude=None):
    super().validate_unique(exclude)
    queryset = type(self).objects.filter(
        value=self.value,
        foreign__parent=self.foreign.parent,
    )

    if self.pk:
        queryset = queryset.exclude(pk=self.pk)

    if queryset.exists():
        raise IntegrityError(_('Value must be unique for foreign field'))

def save(self, force_insert=False, force_update=False, using=None,
         update_fields=None):
    with lock_table(type(self)):
        self.validate_unique()
        super().save(force_insert, force_update, using, update_fields)

Should work fairly save.

Django: unique_together for foreign field

2 Answers2