def add_rel(self, source_id, target_id, rel):
        """ Add a relationship: source, target must already exist (see add_node)
            'rel' is the name of the relationship 'contains' or whatever.
            Args:
                source_id: the unique node_id of the source
                target_id: the unique node_id of the target
                rel: name of the relationship
            Returns:
                Nothing
        """
        self.neo_db.add_rel(source_id, target_id, rel)