  
  class Child6(Child4):
      value = models.IntegerField()
  
  
  class Child7(Parent):
      objects = models.Manager()
  
  
  # RelatedManagers
 
</s> remove                 # Force this manager to be the first and thus default
                managers_mapping[default_manager_name] = (0, models.Manager())
            for manager in model._meta.managers:
                if manager.name == "_base_manager" or not manager.use_in_migrations:
                    continue
                reconstruct_manager(manager)
            # Sort all managers by their creation counter but take only name and
            # instance for further processing
            managers = [
                (name, instance) for name, (cc, instance) in
                sorted(managers_mapping.items(), key=lambda v: v[1])
            ]
            # If the only manager on the model is the default manager defined
            # by Django (`objects = models.Manager()`), this manager will not
            # be added to the model state.
            if managers == [('objects', models.Manager())]:
                managers = []
        else:
            managers = []
 </s> add                 default_manager = models.Manager()
                default_manager.model = model
                default_manager.name = model._default_manager.name
            managers.append((force_text(default_manager.name), default_manager))

        for manager in model._meta.managers:
            if manager.use_in_migrations and manager is not model._default_manager:
                manager = copy.copy(manager)
                manager._set_creation_counter()
                managers.append((force_text(manager.name), manager)) </s> remove         orig_class = Article._base_manager.__class__
 </s> add         orig_class = Article._base_manager._queryset_class </s> remove             Article._base_manager.__class__ = FakeManager
 </s> add             Article._base_manager._queryset_class = FakeQuerySet
    class Meta:
        manager_inheritance_from_future = True

  
  class Child6(Child4):
      value = models.IntegerField()
  
    class Meta:
        manager_inheritance_from_future = True

  
  class Child7(Parent):
      objects = models.Manager()
  
  
  # RelatedManagers
keep keep keep add keep keep 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/managers_regress/models.py