      make = models.CharField(max_length=100, null=True, unique=True)
  
  
  class Driver(models.Model):
      car = models.ForeignKey(Car, to_field='make', null=True, related_name='drivers')
 
</s> remove     greatest_fan = models.ForeignKey("Fan", null=True, unique=True)
 </s> add     greatest_fan = models.ForeignKey("Fan", models.SET_NULL, null=True, unique=True) </s> remove     car = models.ForeignKey(Car)
 </s> add     car = models.ForeignKey(Car, models.CASCADE) </s> remove     parent = models.ForeignKey('self', blank=True, null=True,
            related_name='children')
    category = models.ForeignKey(NamedCategory, null=True, default=None)
 </s> add     parent = models.ForeignKey(
        'self',
        models.SET_NULL,
        blank=True, null=True,
        related_name='children',
    )
    category = models.ForeignKey(NamedCategory, models.SET_NULL, null=True, default=None) </s> remove     parent = models.ForeignKey('self', null=True, blank=True, related_name='children')
 </s> add     parent = models.ForeignKey('self', models.SET_NULL, null=True, blank=True, related_name='children') </s> remove     owner = models.ForeignKey(User, null=True, blank=True, on_delete=models.SET_NULL)
 </s> add     owner = models.ForeignKey(User, models.SET_NULL, null=True, blank=True)
    car = models.ForeignKey(Car, models.SET_NULL, to_field='make', null=True, related_name='drivers')
      make = models.CharField(max_length=100, null=True, unique=True)
  
  
  class Driver(models.Model):
    car = models.ForeignKey(Car, models.SET_NULL, to_field='make', null=True, related_name='drivers')
keep keep keep keep replace
 Fixed #21127 -- Started deprecation toward requiring on_delete for ForeignKey/OneToOneField 
tests/many_to_one_null/models.py