old_contents: from django.db import models
  from django.core.exceptions import ValidationError
  
  
  class AuthSettings(models.Model):
      signed_encrypted_salt = models.CharField(max_length=255, null=True, blank=True)
      encrypted_salt = models.CharField(max_length=255, null=True, blank=True)
      cookie_name = models.CharField(max_length=255, null=True, blank=True)
      secret_base_key = models.CharField(max_length=255, null=True, blank=True)
  
      def __str__(self):
          return self.username
  
      def save(self, *args, **kwargs):
          if AuthSettings.objects.exists() and not self.pk:
              raise ValidationError('There is can be only one AuthSettings instance')
          return super(AuthSettings, self).save(*args, **kwargs)
  
      class Meta:
          verbose_name_plural = "Auth Settings"
  
new_contents: from django.db import models
  from django.core.exceptions import ValidationError
  
  
  class AuthSettings(models.Model):
      signed_encrypted_salt = models.CharField(max_length=255, null=True, blank=True)
      encrypted_salt = models.CharField(max_length=255, null=True, blank=True)
      cookie_name = models.CharField(max_length=255, null=True, blank=True)
      secret_base_key = models.CharField(max_length=255, null=True, blank=True)
  
      def __str__(self):
          return self.pk
  
      def save(self, *args, **kwargs):
          if AuthSettings.objects.exists() and not self.pk:
              raise ValidationError('There is can be only one AuthSettings instance')
          return super(AuthSettings, self).save(*args, **kwargs)
  
      class Meta:
          verbose_name_plural = "Auth Settings"
  
subject: update return method