          )
  
      def test_related_object(self):
          public_school = School.objects.create(is_public=True)
          public_director = Director.objects.create(school=public_school, is_temp=False)
  
 
</s> remove         model = self.rel.related_model if self.reverse else self.rel.model
 </s> add         related_model = self.rel.related_model if self.reverse else self.rel.model
 </s> remove         manager = self.field.remote_field.model._default_manager
        # If the related manager indicates that it should be used for
        # related fields, respect that.
        if not getattr(manager, 'use_for_related_fields', False):
            manager = self.field.remote_field.model._base_manager
 </s> add         related_model = self.field.remote_field.model

        if (not related_model._meta.base_manager_name and
                getattr(related_model._default_manager, 'use_for_related_fields', False)):
            if not getattr(related_model._default_manager, 'silence_use_for_related_fields_deprecation', False):
                warnings.warn(
                    "use_for_related_fields is deprecated, instead "
                    "set Meta.base_manager_name on '{}'.".format(related_model._meta.label),
                    RemovedInDjango20Warning, 2
                )
            manager = related_model._default_manager
        else:
            manager = related_model._base_manager
 </s> remove         manager = self.related.related_model._default_manager
        # If the related manager indicates that it should be used for
        # related fields, respect that.
        if not getattr(manager, 'use_for_related_fields', False):
            manager = self.related.related_model._base_manager
 </s> add         related_model = self.related.related_model

        if (not related_model._meta.base_manager_name and
                getattr(related_model._default_manager, 'use_for_related_fields', False)):
            if not getattr(related_model._default_manager, 'silence_use_for_related_fields_deprecation', False):
                warnings.warn(
                    "use_for_related_fields is deprecated, instead "
                    "set Meta.base_manager_name on '{}'.".format(related_model._meta.label),
                    RemovedInDjango20Warning, 2
                )
            manager = related_model._default_manager
        else:
            manager = related_model._base_manager

    @ignore_warnings(category=RemovedInDjango20Warning)  # for use_for_related_fields deprecation
          )
  
    @ignore_warnings(category=RemovedInDjango20Warning)  # for use_for_related_fields deprecation
      def test_related_object(self):
          public_school = School.objects.create(is_public=True)
          public_director = Director.objects.create(school=public_school, is_temp=False)
  
keep add keep keep keep keep
 Fixed #10506, #13793, #14891, #25201 -- Introduced new APIs to specify models' default and base managers.

This deprecates use_for_related_fields.

Old API:

class CustomManager(models.Model):
    use_for_related_fields = True

class Model(models.Model):
    custom_manager = CustomManager()

New API:

class Model(models.Model):
    custom_manager = CustomManager()

    class Meta:
        base_manager_name = 'custom_manager'

Refs #20932, #25897.

Thanks Carl Meyer for the guidance throughout this work.
Thanks Tim Graham for writing the docs. 
tests/one_to_one/tests.py