Given a Model class, find all related objects, exploring children
        recursively, returning a `list` of strings representing the
        relations for select_related

def _get_subclasses_recurse(self, model, levels=None):
        """
        Given a Model class, find all related objects, exploring children
        recursively, returning a `list` of strings representing the
        relations for select_related
        """
        related_objects = [
            f for f in model._meta.get_fields()
            if isinstance(f, OneToOneRel)]

        rels = [
            rel for rel in related_objects
            if isinstance(rel.field, OneToOneField)
            and issubclass(rel.field.model, model)
            and model is not rel.field.model
            and rel.parent_link
        ]

        subclasses = []
        if levels:
            levels -= 1
        for rel in rels:
            if levels or levels is None:
                for subclass in self._get_subclasses_recurse(
                        rel.field.model, levels=levels):
                    subclasses.append(
                        rel.get_accessor_name() + LOOKUP_SEP + subclass)
            subclasses.append(rel.get_accessor_name())
        return subclasses