def set_ref(self, ref_key, ref_id):
        """
            Using a ref key and ref id set the
            reference to the appropriate resource type.
        """
        if ref_key == 'NETWORK':
            self.network_id = ref_id
        elif ref_key == 'NODE':
            self.node_id = ref_id
        elif ref_key == 'LINK':
            self.link_id = ref_id
        elif ref_key == 'GROUP':
            self.group_id = ref_id
        elif ref_key == 'SCENARIO':
            self.scenario_id = ref_id
        elif ref_key == 'PROJECT':
            self.project_id = ref_id

        else:
            raise HydraError("Ref Key %s not recognised."%ref_key)
Using a ref key and ref id set the
            reference to the appropriate resource type.