繼承 - 混合模式

通常，選用了一種繼承的映射模式之後，就無法再套用另一種模式，但在某些情況下，您還是可以混用兩種繼承模式，例如混用繼承 - Table per class hierarchy 與繼承 - Table per subclass，用Table per class hierarchy的方式將 DefaultUser 映射至T_USER表格，而用Table per subclass的方式將PowerUser的新定義的屬性映射至另一個T_Poweruser表格。
    create table T_PowerUser (
        id bigint not null,
        otherProperty varchar(255),
        primary key (id)
    )

    create table T_USER (
        id bigint not null auto_increment,
        userType varchar(255) not null,
        name varchar(255),
        someProperty varchar(255),
        primary key (id)
    )

    alter table T_PowerUser
        add index id (id),
        add constraint id
        foreign key (id)
        references T_USER (id)

您可以使用<join>標籤來加以定義，例如：

User.hbm.xml

<?xml version="1.0" encoding="utf-8"?> 
<!DOCTYPE hibernate-mapping 
 PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN" 
 "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd"> 

<hibernate-mapping package="onlyfun.caterpillar"> 

    <class name="User" table="T_USER"> 

        <id name="id" column="id"> 
            <generator class="native"/> 
        </id> 

        <discriminator column="userType"/> 
 
        <property name="name" column="name"/> 

        <subclass name="DefaultUser" 
                  discriminator-value="Default"> 
            <property name="someProperty" column="someProperty" /> 
        </subclass> 
 
        <subclass name="PowerUser" 
                  discriminator-value="Power"> 
           <join table="T_PowerUser">
		<key column="id" foreign-key="id"/>
                <property name="otherProperty" column="otherProperty"/>
           </join> 
        </subclass>  

    </class> 

</hibernate-mapping>

儲存物件時的一個例子如下：

Hibernate:
    insert
    into
        T_USER
        (name, userType)
    values
        (?, 'Power')
Hibernate:
    insert
    into
        T_PowerUser
        (otherProperty, id)
    values
        (?, ?)
Hibernate:
    insert
    into
        T_USER
        (name, someProperty, userType)
    values
        (?, ?, 'Default')

查詢時的一個例子如下：

Hibernate:
    select
        user0_.id as id0_,
        user0_.name as name0_,
        user0_.someProperty as someProp4_0_,
        user0_1_.otherProperty as otherPro2_1_,
        user0_.userType as userType0_
    from
        T_USER user0_
    left outer join
        T_PowerUser user0_1_
            on user0_.id=user0_1_.id

儲存的表格範例如下：

這個混用模式的問題是，在繼承的廣度大時，outer join會成為問題，您可以在<join>上加上fetch屬性為select：
...
<join table="T_PowerUser" fetch="select">
                <key column="id" foreign-key="FK_USER"/>
                <property name="otherProperty" column="otherProperty"/>
            </join>
...

則查詢時的一個例子如下，原來的一次查詢換成了兩次select查詢：

Hibernate:
    select
        user0_.id as id0_,
        user0_.name as name0_,
        user0_.someProperty as someProp4_0_,
        user0_.userType as userType0_
    from
        T_USER user0_
Hibernate:
    select
        user_1_.otherProperty as otherPro2_1_
    from
        T_PowerUser user_1_
    where
        user_1_.id=?