Empty string validator not run on nullable charfield

[thenewguy]

I need to store a CharField as null for unique together. I've hit an oddity attempting to prevent empty strings from passing validation.

I would expect for test_cannot_create_for_session_with_empty_string() below to pass. It fails. However, test_cannot_create_altfoo_for_session_with_empty_string() does pass.

def validate_truthy_or_null(value):
    if not value and value is not None:
        raise ValidationError('"%(value)s" was not truthy or null', params={'value': value})
​
​
class AbstractFoo(models.Model):
    class Meta:
        abstract = True
		
    session = models.CharField(
        max_length=255,
        null=True,
        blank=True,
        default=None,
        editable=False,
        validators=[validate_truthy_or_null],
    )


class Foo(AbstractFoo):
	pass


class AltFoo(AbstractFoo)
	def clean(self):
        validate_truthy_or_null(self.session)
​
​
class DemonstrationTests(TestCase):
    def test_validate_truthy_or_null(self):
        for value in ('', 0, False):
            with self.subTest(value=value):
                with self.assertRaises(ValidationError):
                    validate_truthy_or_null(value)
​
    def test_cannot_create_for_session_with_empty_string(self):
        foo = Foo()
        foo.session = ''
        with self.assertRaises(ValidationError):
            foo.full_clean()
	
    def test_cannot_create_altfoo_for_session_with_empty_string(self):
        alt = AltFoo()
        alt.session = ''
        with self.assertRaises(ValidationError):
            alt.full_clean()

[thenewguy]

Closed. Looks like issue is editable=False per ​https://docs.djangoproject.com/en/3.0/ref/models/fields/#editable