hibernate继承映射

每一个实体类都有一些对象，对象是数据的载体，每个对象在数据库中可能是一条或多条数据。而实体类之间存在继承关系，所以在数据库中表格之间也存在相应的继承关系。强大的对象关系管理（ORM）hibernate,也提供了三种方式的解决方案。

1. 一个类对应一张表<union-subclass>
2. 一个子类对应一张分表<joined-subclass>
3. 一个继承体系对应一张表

我们将根据下面这个类的关系做具体讲解：

• 一个子类对应一张表<union-subclass>
  每一个子类对应的数据库表都包含了父类的信息，并且包含了自己独有的属性。每个子类对应一张表，而且这个表的信息是完备的，即包含了所有从父类继承下来的属性映射的字段。

1. Person.hbm.xml

<hibernate-mapping package="hibernate_3_extend" >
    <class name="Person" table="t_Person">
        <id name="id" >
            <generator class="uuid"/>
        </id>
        <property name="name" type="string" column="name" length="20"/>
        
        <union-subclass name="hibernate_3_extend.Student" table="t_student" >
            <property name="major" type="string" column="major" length="20"></property>
        </union-subclass>
        <union-subclass name="hibernate_3_extend.Teacher" table="t_teacher" >
            <property name="salary" type="float" column="salary"></property>
        </union-subclass>
    </class>
</hibernate-mapping>

2. teacher.sql

 create table t_teacher (
  id integer not null,
  name varchar(20),
  salary float, 
  primary key (id)
) engine=InnoDB

3. student.sql

create table t_student (
  id integer not null,
  name varchar(20),
  major varchar(20), 
  primary key (id)
) engine=InnoDB

4. person.sql

create table t_Person (
  id integer not null,
  name varchar(20),
  primary key (id)
) engine=InnoDB

• 一个子类对应一张分表<joined-subclass>

每一个子类对应的数据库表只包含了自己独有的属性。每个子类对应一张表，而且这个表的信息是不完备的。

1. person.hbm.xml

<hibernate-mapping package="hibernate_3_extend" >
    <class name="Person" table="t_Person">
        <id name="id" >
            <generator class="uuid"/>
        </id>
        <property name="name" type="string" column="name" length="20"/>
        
        <joined-subclass name="hibernate_3_extend.Student" table="t_student">
            <key column="id"></key>
            <property name="major" type="string" column="major" length="20"></property>
        </joined-subclass>
        
        <joined-subclass name="hibernate_3_extend.Teacher" table="t_teacher">
            <key column="id"></key>
            <property name="salary" type="float" column="salary"></property>
        </joined-subclass>
    </class>
</hibernate-mapping>

2. person.sql

create table t_Person (
 id integer not null,
 name varchar(20),
 primary key (id)
) engine=InnoDB

3. student.sql

create table t_student (
 id integer not null, 
 major varchar(20), 
 primary key (id)
) engine=InnoDB

4. teacher.sql

create table t_teacher (
 id integer not null, 
 salary float, 
 primary key (id)
) engine=InnoDB

5. 最后添加外键

alter table t_student add constraint FK1dntcikk4ddyfgyfif10kwp9c foreign key (id) references t_Person (id)
alter table t_teacher add constraint FKc2hk0m55tq5nmse34dhgyd5gc foreign key (id) references t_Person (id)

• 一个继承体系对应一张表<subclass>
  这种关系通过<discriminator>标签来实现，区分类和角色，增加一个字段

1. person.hbm.xml

<hibernate-mapping package="hibernate_3_extend" >
    <class name="Person" table="t_Person">
        <id name="id"  >
            <generator class="uuid"/>
        </id>

        <discriminator column="type"  type="string"></discriminator>
    
        <property name="name" type="string" column="name" length="20"/>
        
        <subclass name="hibernate_3_extend.Student"  discriminator-value="student">
            <property name="major" type="string" column="major" length="20"></property>
        </subclass>
        <subclass name="hibernate_3_extend.Teacher" discriminator-value="teacher">
            <property name="salary" type="float" column="salary"></property>
        </subclass>
    </class>
</hibernate-mapping>

2. person.sql

create table t_Person (
 id integer not null, 
 type varchar(255) not null, 
 name varchar(20), 
 major varchar(20), 
 salary float, 
 primary key (id)
) engine=InnoDB