Hibernate两个列作为唯一索引

<hibernate-mapping package="hjds.domain.privilege">
    <class name="AdminRole" table="adminroles" catalog="hjds">
        <id name="id">
            <generator class="native"/>
        </id>
        <many-to-one name="admin" class="Admin" column="aid" cascade="save-update" unique-key="tt"/>
        <many-to-one name="role" class="Role" column="rid" cascade="save-update" unique-key="tt"/>
    </class>
</hibernate-mapping>