唯一的共同约束，包括特定的字段值

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'm looking forward something like

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

    class Meta:
        unique_together = (('field_a', 'field_b', 'validated=True'),)

Answer 1

You can use UniqueConstraint in case you're using Django 2.2+ Here is an example

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

    class Meta:
        constraints = [
            UniqueConstraint(fields=['field_a', 'field_b'], condition=Q(validated=True), name='unique_field_a_field_b_validated')
        ]

here is the source

Answer 2

(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, ie

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, ie

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

Answer 3

In addition to the previous answer you can overwrite the save() method. It would be something like this:

def save(self, **kwargs):
    try:
        self.objects.get(field_a=self.field_a, field_b=self.field_b, validated=True)

        # The object already exist therefore throw an exception
        raise ValidationError(
            "field_a and field_b must be unique if validated=True"
        ) 

    except self.__class__.DoesNotExist:  # Save the model
        super(MyModel, self).save(**kwargs)  # inherit and call the save method

Now you don't need to call the clean() method.