      To test postgres-specific aggregation functions for statistics
      """
      int1 = models.IntegerField()
      int2 = models.IntegerField()
      related_field = models.ForeignKey(AggregateTestModel, null=True)
  
  
  class NowTestModel(models.Model):
      when = models.DateTimeField(null=True, default=None)
 
</s> remove     self_data = models.ForeignKey('self', null=True)
 </s> add     self_data = models.ForeignKey('self', models.SET_NULL, null=True) </s> remove     holder = models.ForeignKey(Holder2)
 </s> add     holder = models.ForeignKey(Holder2, models.CASCADE) </s> remove     author = models.ForeignKey(Author, related_name='books', null=True)
 </s> add     author = models.ForeignKey(Author, models.SET_NULL, related_name='books', null=True) </s> remove     profile = models.ForeignKey(UserProfile, to_field="user")
 </s> add     profile = models.ForeignKey(UserProfile, models.CASCADE, to_field="user") </s> remove     holder = models.ForeignKey(Holder3)
 </s> add     holder = models.ForeignKey(Holder3, models.CASCADE)
    related_field = models.ForeignKey(AggregateTestModel, models.SET_NULL, null=True)
      To test postgres-specific aggregation functions for statistics
      """
      int1 = models.IntegerField()
      int2 = models.IntegerField()
    related_field = models.ForeignKey(AggregateTestModel, models.SET_NULL, null=True)
  
  
  class NowTestModel(models.Model):
      when = models.DateTimeField(null=True, default=None)
keep keep keep keep replace keep keep keep keep
 Fixed #21127 -- Started deprecation toward requiring on_delete for ForeignKey/OneToOneField 
tests/postgres_tests/models.py