当我用另一个对象更新时，hibernate不会更新鉴别符列 [英] hibernate doesn't update discriminator column when I update with another object

问题描述

我在Hibernate中使用了每个层次结构的类策略"来映射以下类层次结构.

I have used a "class per hierarchy strategy" for mapping the following class hierarchy in Hibernate.

我有 Item 作为抽象类. 具体如下: 图书 日记 DVD CD

I have Item as an abstract class. The followings as concrete ones: Book Journal DVD CD

我的映射非常适合保存和加载，但是在更新具有不同类的项目时无法正常工作. 我在上述每个类中都使用了区分符.

My mapping works perfect for saving and loading but it doesn't work correctly on updating an item with a different class. I'm using discriminators for each of the above classes.

我的item.hbm.xml就像:

my item.hbm.xml is like:

<!-- language: lang-xml -->
<hibernate-mapping>
    <class name="my.hibernate.items.Item" discriminator-value="item"
        table="testitem" catalog="efeu">
        <id name="id" type="java.lang.Long">
            <column name="id" />
            <generator class="native" />
        </id>
        <discriminator column="item_type" type="string" length="8" />
        <subclass name="my.hibernate.items.DVD" discriminator-value="DVD">
            <property name="duration" type="java.lang.Integer" column="duration" />
            <property name="trackNumber" type="java.lang.Integer"
                column="trackNumber" />
        </subclass>
        <subclass name="my.hibernate.items.Journal"
            discriminator-value="Journal">
            <property name="serialNumber" type="java.lang.Integer"
                column="serial_number" />
        </subclass>
    </class>
</hibernate-mapping>

对于更新，我使用此方法:

and for updating I use this method:

<!-- language: lang-java -->
public static void updateItem(Item newItem){
    Session session = HibernateSessionFactory.getSession();
    Transaction tx = session.beginTransaction();

    session.update(newItem);// I tried merge too. it created a new row

    tx.commit();
    session.close();
}

我将哪个类实例发送到updateItem方法没有区别.除区分符外，所有字段都将更新，因此，如果它是一本书，那么它仍然是一本书，因此我无法将其以此类对象作为参数调用上述方法，将其转换为CD，DVD或日记.

There is no difference which class instance I send to updateItem method. All the fields are updated except the discriminator so if it is a book remains a book and I can't convert it to CD, DVD or journal invoking the above method with an object of that kind as parameter.

我认为我在做什么很明显.我假设我知道数据库表中id = 111的行是Book对象.因此，我执行以下操作:

I think it is clear what I am doing. I assum that I know that the row with id=111 in the database table is a Book object. So I do the following:

<!-- language: lang-java -->
Item cd = new CD();
cd.setId(111);
cd.setXXXProperties()
Item.updateItem(cd);

很明显，在调用该方法之前cd是瞬态的，在此之前我还没有加载id = 111的行.

it is clear that cd is transient before invoking the method and I haven't loaded the row with id=111 before that.

在调用更新方法时，如何使Hibernate更改discriminator列?

What should I do to make Hibernate change the discriminator column on invoking the update method?

推荐答案

无法更新discriminator值.您所遇到的是正确的.一旦创建了对象(作为CD或书籍)，它就不会更改其类型.

There is no way how to update a discriminator value. What you are experiencing is correct. Once object is created (as a CD or as a Book) it won't change its type.

在此处了解更多信息: 5.1 .6.1.1.鉴别符

Read more here: 5.1.6.1.1. Discriminator

<discriminator
        column="discriminator_column"                      (1)
        type="discriminator_type"                          (2)
        force="true|false"                                 (3)
        insert="true|false"                                (4)
        formula="arbitrary sql expression"                 (5)
/>

没有针对鉴别器的更新操作.

There is no update operation for discriminator.

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