Hibernate的继承映射

                                       Hibernate的继承映射

        在一个类层次映射成一个表时，定义的discriminator只在hibernate与数据库之间使用。在持久化类中并不需要定义discriminator字段，即使定义了，而且数据库中该字段不为空，程序中访问该字段也将得到null值。也就是说，discriminator对应用程序是不可见的。从DTD里也可以看出，discriminator没有name元素，也就是说持久化类中不必为相应column定义一个属性。hibernate根据对象的具体类并参照映射文件中的discriminator字段来确定数据库中记录的更新、保存以及加载。采用这种策略只需要一张表即可。它有一个很大的限制：要求那些由子类定义的字段，不能有非空(NOT NULL)约束。

        在每个子类一张表时，连同超类，按照下面的实例，数据库中需要三张表。子类表通过主键关联到超类表(因而关系模型实际上是一对一关联)。子类中 的idPayment在数据库中可以设置成主键。

1、一个类层次对应一个表

< hibernate-mapping >
     < class  name ="com.persistent.Payment"  table ="payment"   >     
         < id  name ="id"  column ="paymentId"  unsaved-value ="null" >
             < generator  class ="increment" ></ generator >
         </ id >         
         < discriminator  column ="paymentType" ></ discriminator >         
         < property  name ="amount"  column ="amount" ></ property >         
         < subclass  name ="com.persistent.CashPayment"  discriminator-value ="cash" >
             < property  name ="cashType"  column ="cashType" ></ property >
         </ subclass >         
         < subclass  name ="com.persistent.CardPayment"  discriminator-value ="card" >
             < property  name ="cardNumber"  column ="cardNumber" ></ property >
         </ subclass >     
     </ class >
</ hibernate-mapping >

2、每个子类一张表

< hibernate-mapping >
     < class  name ="com.persistent.Payment"  table ="payment"   >     
         < id  name ="id"  column ="paymentId"  unsaved-value ="null" >
             < generator  class ="increment" ></ generator >
         </ id >         
         < property  name ="amount"  column ="amount" ></ property >         
         < joined-subclass  name ="com.persistent.CashPayment"  table ="cashpayment" >
             < key  column ="idPayment" ></ key >
             < property  name ="cashType"  column ="cashType" ></ property >
         </ joined-subclass >         
         < joined-subclass  name ="com.persistent.CardPayment"  table ="cardpayment" >
             < key  column ="idPayment" ></ key >
             < property  name ="cardNumber"  column ="cardNumber" ></ property >
         </ joined-subclass >     
     </ class >
</ hibernate-mapping >