Django unique_together work with ForeignKey=None

gistfile1.py

class FooBar(models.Model):
 foo = models.ForeignKey(Foo)
 bar = models.ForeignKey(Bar, null=True, blank=True)

 class Meta:
  unique_together = ('foo', 'bar')

 def clean(self):
   from django.core.exceptions import ValidationError
   if self.foo and FooBar.objects.filter(name=self.bar).exists():
     raise ValidationError("...")