Django数据库错误：关系已经存在

Question

我有一个模特：

class Tag(models.Model):
name = models.CharField(max_length=30)
parent = models.ManyToManyField("self", null=True, blank=True)

def __str__(self):
    return '{}'.format(self.name)

def clean(self):
    if self.name:
        if self.name == 'Minor Locality' and self.parent != 'City':
            raise ValidationError({
            NON_FIELD_ERRORS: ["Parent tag can't be other City tag "]
        })
        if self.name == 'Locality' and self.parent != 'City':
            raise ValidationError({
            NON_FIELD_ERRORS: ["Parent tag can't be other City tag "]
        })


def save(self, *args, **kwargs):
    self.clean()
    super(Tag, self).save(*args, **kwargs)

当我尝试添加标记“City”并将其父级保留为空白时，会在保存时抛出错误：

ProgrammingError at /admin/locality/tag/add/

relation "locality_tag_parent" does not exist
LINE 1: DELETE FROM "locality_tag_parent" WHERE ("locality_tag_paren...