hibernate的继承类的映射

Root.java{Long id;Long Version;String code}                                                                                         

Child1.java extends Root{String a;}

Child2.java extends Roor{String b;}                       

1.直接映射，2个类Child1，Child2分别映射2个表

<hibernate-mapping>
    <class name="Root" table="CHILD1">
        <id name="id" column="CHILD1_ID" type="long">
            <generator class="native">
                <param name="sequence">root_sequence</param>
                <param name="parameters">START WITH 1000</param>
            </generator>
        </id>
        <version name="version" column="VERSION" type="long"/>
        <property name="code" type="string">
            <column name="CODE" not-null="true" length="100" unique-key="UK_ROOT"/>
        </property>
        <property name="a" column="A" type="string"></property>  
    </class>
</hibernate-mapping>

<hibernate-mapping>
    <class name="Root" table="CHILD2">
        <id name="id" column="CHILD2_ID" type="long">
            <generator class="native">
                <param name="sequence">root_sequence</param>
                <param name="parameters">START WITH 1000</param>
            </generator>
        </id>
        <version name="version" column="VERSION" type="long"/>
        <property name="code" type="string">
            <column name="CODE" not-null="true" length="100" unique-key="UK_ROOT"/>
        </property>
        <property name="a" column="A" type="string"></property>  
    </class>
</hibernate-mapping>

这是最简单的映射策略，隐式多态，查询时面向对象，查询Root，会把其子类Child1，Child2的表都搜索

2。有joined-subclass，将公共字段放在一张表root上，child1表上存id，a；child2表存id，b；其中id字段与root上的字段关联；共3张表

<hibernate-mapping>
    <class name="Root" table="ROOT">
        <id name="id" column="ROOT_ID" type="long">
            <generator class="native">
                <param name="sequence">root_sequence</param>
                <param name="parameters">START WITH 1000</param>
            </generator>
        </id>
        <version name="version" column="VERSION" type="long"/>
        <property name="code" type="string">
            <column name="CODE" not-null="true" length="100" unique-key="UK_ROOT"/>
        </property>
        <joined-subclass name="Child1" table="CHILD1">
            <key column="id"></key>
            <property name="a" type="string">
                <column name="A" not-null="true" unique-key="UK_CHILD1"/>
            </property>
        </joined-subclass>
        <joined-subclass name="Child2" table="CHILD2">
            <key column="id"></key>
            <property name="b" type="string">
                <column name="B" not-null="true" unique-key="UK_CHILD2"/>
            </property>
        </joined-subclass>
    </class>
</hibernate-mapping>

其中joined-subclass下的key表示外键，查询Child1的时候，会将root表与child1联合，效率也不高

3.采用discriminator，只有一张表

<hibernate-mapping>
    <class name="Root" table="ROOT">
        <id name="id" column="ROOT_ID" type="long">
            <generator class="native">
                <param name="sequence">root_sequence</param>
                <param name="parameters">START WITH 1000</param>
            </generator>
        </id>
        <discriminator  column="TYPE" type="string"></discriminator>
        <version name="version" column="VERSION" type="long"/>
        <property name="code" type="string">
            <column name="CODE" not-null="true" length="100" unique-key="UK_ROOT"/>
        </property>
        <subclass name="Child1" discriminator-value="1">
            <property name="a" column="A" type="string"></property>
        </subclass>
        <subclass name="Child2" discriminator-value="2">
            <property name="b" column="B" type="string"></property>
        </subclass>
    </class>
</hibernate-mapping>

注意的是discriminator 要在id和version之间，而且hbm文件都必须有dtd。这样的查询效率高多了，查询child1的时候会自动根据条件type＝1