def refine_many_to_one_related_manager(ctx: MethodContext) -> MypyType:
    """
    Updates the 'RelatedManager' returned by e.g. 'ReverseManyToOneDescriptor' to be a subclass
    of 'RelatedManager' and the related model's default manager.
    """
    to_model_instance = get_model_of_related_manager(ctx)
    if to_model_instance is None:
        return ctx.default_return_type

    checker = helpers.get_typechecker_api(ctx)
    related_manager_info = helpers.get_reverse_manager_info(
        checker, to_model_instance.type, derived_from="_default_manager"
    )
    if related_manager_info is None:
        return ctx.default_return_type
    return Instance(related_manager_info, [])

    Updates the 'RelatedManager' returned by e.g. 'ReverseManyToOneDescriptor' to be a subclass
    of 'RelatedManager' and the related model's default manager.
    
mypy_django_plugin/transformers/manytoone.py