  example above::
  
      class Book(models.Model):
          name = models.CharField(max_length=100)
          author = models.ForeignKey(Person)
  
          def natural_key(self):
              return (self.name,) + self.author.natural_key()
  
  The natural key for a ``Book`` is a combination of its name and its
 
</s> remove         author = models.ForeignKey(Person)
 </s> add         author = models.ForeignKey(Person, on_delete=models.CASCADE) </s> remove     user = models.OneToOneField(User, primary_key=True)
 </s> add     user = models.OneToOneField(User, models.CASCADE, primary_key=True) </s> remove         cat = models.ForeignKey(Category)
 </s> add         cat = models.ForeignKey(Category, models.CASCADE) </s> remove         author = models.ForeignKey(Author)
 </s> add         author = models.ForeignKey(Author, on_delete=models.CASCADE) </s> remove     parent = models.ForeignKey(ParentWithDependentChildren)
 </s> add     parent = models.ForeignKey(ParentWithDependentChildren, models.CASCADE)
        author = models.ForeignKey(Person, on_delete=models.CASCADE)
  example above::
  
      class Book(models.Model):
          name = models.CharField(max_length=100)
        author = models.ForeignKey(Person, on_delete=models.CASCADE)
  
          def natural_key(self):
              return (self.name,) + self.author.natural_key()
  
  The natural key for a ``Book`` is a combination of its name and its
keep keep keep keep replace keep keep keep keep keep
 Fixed #21127 -- Started deprecation toward requiring on_delete for ForeignKey/OneToOneField 
docs/topics/serialization.txt