def _assign_child_to_parent(self, child: Optional["T"]) -> None:
        """
        Registers child in parents RelationManager.

        :param child: child to register on parent side.
        :type child: Model
        """
        if child:
            owner = self._owner
            rel_name = self.relation.field_name
            setattr(owner, rel_name, child)

        Registers child in parents RelationManager.

        :param child: child to register on parent side.
        :type child: Model
        
ormar/relations/querysetproxy.py