23.2. Author(作家)/Work(作品)

考虑下面的Work,Author 和 Person模型的关系。 我们用多对多关系来描述Work 和 Author， 用一对一关系来描述Author 和 Person， 另一种可能性是Author继承Person。

下面的映射文件正确的描述了这些关系：

<hibernate-mapping>

    <class name="Work" table="works" discriminator-value="W">

        <id name="id" column="id">
            <generator class="native"/>
        </id>
        <discriminator column="type" type="character"/>

        <property name="title"/>
        <set name="authors" table="author_work">
            <key column name="work_id"/>
            <many-to-many class="Author" column name="author_id"/>
        </set>

        <subclass name="Book" discriminator-value="B">
            <property name="text"/>
        </subclass>

        <subclass name="Song" discriminator-value="S">
            <property name="tempo"/>
            <property name="genre"/>
        </subclass>

    </class>

    <class name="Author" table="authors">

        <id name="id" column="id">
            <!-- The Author must have the same identifier as the Person -->
            <generator class="assigned"/> 
        </id>

        <property name="alias"/>
        <one-to-one name="person" constrained="true"/>

        <set name="works" table="author_work" inverse="true">
            <key column="author_id"/>
            <many-to-many class="Work" column="work_id"/>
        </set>

    </class>

    <class name="Person" table="persons">
        <id name="id" column="id">
            <generator class="native"/>
        </id>
        <property name="name"/>
    </class>

</hibernate-mapping>

映射中有4个表。works, authors 和 persons 分别保存着work，author和person的数据。author_work是authors和works的关联表。 表结构是由SchemaExport生成的。

create table works (
    id BIGINT not null generated by default as identity, 
    tempo FLOAT, 
    genre VARCHAR(255), 
    text INTEGER, 
    title VARCHAR(255), 
    type CHAR(1) not null, 
    primary key (id)
)

create table author_work (
    author_id BIGINT not null, 
    work_id BIGINT not null, 
    primary key (work_id, author_id)
)

create table authors (
    id BIGINT not null generated by default as identity, 
    alias VARCHAR(255), 
    primary key (id)
)

create table persons (
    id BIGINT not null generated by default as identity, 
    name VARCHAR(255), 
    primary key (id)
)

alter table authors 
    add constraint authorsFK0 foreign key (id) references persons
alter table author_work 
    add constraint author_workFK0 foreign key (author_id) references authors
alter table author_work
    add constraint author_workFK1 foreign key (work_id) references works