simple unique_together check with M2M field

class MyModel(models.Model):
    site = models.ManyToManyField(Site)
    on_site = CurrentSiteManager()
    
    foobar = ... # A unique field
    
    def save(self, *args, **kwargs):
        if self.id == None: # new item should be created.
            # manually check a unique togeher, because django can't do this with a M2M field.
            # Obsolete if unique_together work with ManyToMany: http://code.djangoproject.com/ticket/702
            exist = MyModel.on_site.filter(foobar=self.foobar).count()
            if exist != 0:
                from django.db import IntegrityError
                # We can use attributes from this model instance, because it needs to have a primary key
                # value before a many-to-many relationship can be used.
                site = Site.objects.get_current()
                raise IntegrityError(
                    "MyModel item with same foobar field exist on site %r" % site
                )
        
        return super(MyModel, self).save(*args, **kwargs)

    class Meta:
        #unique_together = ("foobar", "site") # unique_together doesn't work with ManyToMany!
        # See: http://code.djangoproject.com/ticket/702