  @python_2_unicode_compatible
  class SuperSecretHideout(models.Model):
      """ Secret! Not registered with the admin! """
      location = models.CharField(max_length=100)
      supervillain = models.ForeignKey(SuperVillain)
  
      def __str__(self):
          return self.location
  
  
 
</s> remove     villain = models.ForeignKey(Villain)
 </s> add     villain = models.ForeignKey(Villain, models.CASCADE) </s> remove     honeycomb = models.ForeignKey(Honeycomb)
 </s> add     honeycomb = models.ForeignKey(Honeycomb, models.CASCADE) </s> remove     location = models.ForeignKey(Location)
 </s> add     location = models.ForeignKey(Location, models.CASCADE) </s> remove     car = models.ForeignKey(Car)
 </s> add     car = models.ForeignKey(Car, models.CASCADE) </s> remove     site = models.ForeignKey(Site, related_name="admin_articles")
 </s> add     site = models.ForeignKey(Site, models.CASCADE, related_name="admin_articles")
    supervillain = models.ForeignKey(SuperVillain, models.CASCADE)
  @python_2_unicode_compatible
  class SuperSecretHideout(models.Model):
      """ Secret! Not registered with the admin! """
      location = models.CharField(max_length=100)
    supervillain = models.ForeignKey(SuperVillain, models.CASCADE)
  
      def __str__(self):
          return self.location
  
  
keep keep keep keep replace keep keep keep keep keep
 Fixed #21127 -- Started deprecation toward requiring on_delete for ForeignKey/OneToOneField 
tests/admin_views/models.py