def _register_related(self, child: Union["T", Sequence[Optional["T"]]]) -> None:
        """
        Registers child/ children in parents RelationManager.

        :param child: child or list of children models to register.
        :type child: Union[Model,List[Model]]
        """
        if isinstance(child, list):
            for subchild in child:
                self._assign_child_to_parent(subchild)
        else:
            assert isinstance(child, ormar.Model)
            child = cast("T", child)
            self._assign_child_to_parent(child)

        Registers child/ children in parents RelationManager.

        :param child: child or list of children models to register.
        :type child: Union[Model,List[Model]]
        
ormar/relations/querysetproxy.py