Returns the first (and possibly only) edge connecting node_a and node_b.

def get_first_edge_id_by_node_ids(self, node_a, node_b):
        """Returns the first (and possibly only) edge connecting node_a and node_b."""
        ret = self.get_edge_ids_by_node_ids(node_a, node_b)
        if not ret:
            return None
        else:
            return ret[0]