Unique fields that allow nulls in Django

Question

I have model Foo which has field bar. The bar field should be unique, but allow nulls in it, meaning I want to allow more than one record if bar field is null,

Answer 1

You can add UniqueConstraint with condition of nullable_field=null and not to include this field in fields list. If you need also constraint with nullable_field wich value is not null, you can add additional one.

Note: UniqueConstraint was added since django 2.2

class Foo(models.Model):
    name = models.CharField(max_length=40)
    bar = models.CharField(max_length=40, unique=True, blank=True, null=True, default=None)
    
    class Meta:
        constraints = [
            # For bar == null only
            models.UniqueConstraint(fields=['name'], name='unique__name__when__bar__null',
                                    condition=Q(bar__isnull=True)),
            # For bar != null only
            models.UniqueConstraint(fields=['name', 'bar'], name='unique__name__when__bar__not_null')
        ]