@abc.abstractmethod<EOL><INDENT>def get_relationships_by_query(self, relationship_query):<DEDENT>
return<EOL>
Gets a list of ``Relationships`` matching the given relationship query.

        :param relationship_query: the relationship query
        :type relationship_query: ``osid.relationship.RelationshipQuery``
        :return: the returned ``RelationshipList``
        :rtype: ``osid.relationship.RelationshipList``
        :raise: ``NullArgument`` -- ``relationship_query`` is ``null``
        :raise: ``OperationFailed`` -- unable to complete request
        :raise: ``PermissionDenied`` -- authorization failure
        :raise: ``Unsupported`` -- ``relationship_query`` is not of this service

        *compliance: mandatory -- This method must be implemented.*