Unique together constraint including specific field value

Question

For one of my models, I need to ensure the unicity of some rows, but only in a certain case. Only the \"validated\" rows should follow this constraint.

Basically, I\'

Answer 1

(at time of writing, in Django < 2.2)

You can't do that with unique_together in Django, presumably because not all db backends would be able to support it.

You can do it in the application layer with model validation instead:
https://docs.djangoproject.com/en/dev/ref/models/instances/#validating-objects

eg

class MyModel(models.Model):
    field_a = models.CharField()
    field_b = models.CharField()
    validated = models.BooleanField(default=False)

    def clean(self):
        if not self.validated:
            return
        existing = self.__class__.objects.filter(field_a=self.field_a,
                                                 field_b=self.field_b).count()
        if existing > 0:
            raise ValidationError(
                "field_a and field_b must be unique if validated=True"
            )

Note that you will probably have to call the model validation manually, i.e.

instance.clean()
instance.save()

It is not done automatically when saving the model. On the other hand it is done automatically when using a ModelForm, i.e.

if form.is_valid():
    instance = form.save()