hibernate - 如何将继承策略与 JPA 注解和 Hibernate 混合使用？

Question

根据 Hibernate 引用文档，在使用 Hibernate 的 XML 元数据时应该可以混合不同的继承映射策略:
http://docs.jboss.org/hibernate/stable/core/reference/en/html/inheritance.html#inheritance-mixing-tableperclass-tablepersubclass

但是，Hibernate 注解引用指南的相应部分并未涵盖以下内容:
http://docs.jboss.org/hibernate/stable/annotations/reference/en/html/entity.html#d0e1168

另一方面，JavaDocs 建议混合继承策略应该是可能的。例如在 javax.persistence.DiscriminatorColumn 中它说:

The strategy and the discriminator column are only specified in the root of an entity class hierarchy or subhierarchy in which a different inheritance strategy is applied.

<小时/>

以下是我试图实现的映射的示例。我想在层次结构根部附近使用每个子类一个表映射，但在叶子附近更改为每个类层次结构一个表映射。下面是一些示例代码:

@Entity
@Inheritance( strategy = InheritanceType.JOINED )
public abstract class A implements Serializable
{
    @Id
    private String id;

    // other mapped properties...
}

@Entity
@Inheritance( strategy = InheritanceType.SINGLE_TABLE )
public class BB extends A
{
    // other mapped properties and associations...
}

@Entity
public class BB1 extends BB
{
    // other stuff, not necessarily mapped...
}

@Entity
public class BB2 extends BB
{
    // other stuff, not necessarily mapped...
}

@Entity
@Inheritance( strategy = InheritanceType.SINGLE_TABLE )
public class CC extends A
{
    // other mapped properties and associations...
}

@Entity
public class CC1 extends CC
{
    // other stuff, not necessarily mapped...
}

...

我对该映射的期望是恰好有 3 个表:A、BB 和 CC。 BB 和 CC 都应该有一个名为 DTYPE 的默认鉴别器列。它们还应该提供其各自子类的所有映射属性和关联所需的所有列。

相反，类层次结构似乎自始至终都使用每个子类一个表继承策略。 IE。我为上面提到的每个实体都有一个自己的表。我想避免这种情况，因为类层次结构的叶子非常轻量，并且为每个叶子都有一个单独的表似乎有点过分了!

<小时/>

我是不是忽略了什么？任何建议都将受到高度赞赏!我很乐意提供更多信息...

Answer 1

According to the Hibernate Reference Documentation it should be possible to mix different inheritance mapping strategies when using Hibernate's XML-Metadata (...)

实际上，它并不真正受支持，他们在文档示例中使用辅助表“欺骗”从单表策略切换。引用使用 Hibernate 实现 Java 持久化:

You can map whole inheritance hierarchies by nesting <union-subclass>, <sub- class>, and <joined-subclass> mapping elements. You can’t mix them — for example, to switch from a table-per-class hierarchy with a discriminator to a normalized table-per-subclass strategy. Once you’ve made a decision for an inheritance strategy, you have to stick to it.

This isn’t completely true, however. With some Hibernate tricks, you can switch the mapping strategy for a particular subclass. For example, you can map a class hierarchy to a single table, but for a particular subclass, switch to a separate table with a foreign key mapping strategy, just as with table per subclass. This is possible with the <join> mapping element:

<hibernate-mapping>
  <class name="BillingDetails"
      table="BILLING_DETAILS">

    <id>...</id>

    <discriminator
        column="BILLING_DETAILS_TYPE"
        type="string"/>
    ...
    <subclass
        name="CreditCard"
        discriminator-value="CC">
      <join table="CREDIT_CARD">
        <key column="CREDIT_CARD_ID"/>

        <property name="number" column="CC_NUMBER"/>
        <property name="expMonth" column="CC_EXP_MONTH"/>
        <property name="expYear" column="CC_EXP_YEAR"/>
        ...
      </join>
    </subclass>

    <subclass
        name="BankAccount"
        discriminator-value="BA">
      <property name=account" column="BA_ACCOUNT"/>
      ...
    </subclass>
  ...
  </class>
</hibernate-mapping>

您可以通过注释实现相同的目的:

Java Persistence also supports this mixed inheritance mapping strategy with annotations. Map the superclass BillingDetails with InheritanceType.SINGLE_TABLE, as you did before. Now map the subclass you want to break out of the single table to a secondary table.

@Entity
@DiscriminatorValue("CC")
@SecondaryTable(
    name = "CREDIT_CARD",
    pkJoinColumns = @PrimaryKeyJoinColumn(name = "CREDIT_CARD_ID")
)
public class CreditCard extends BillingDetails {
    @Column(table = "CREDIT_CARD",
        name = "CC_NUMBER",
        nullable = false)
    private String number;
    ...
}

我没有测试这个，但你也许可以尝试:

• 使用 SINGLE_TABLE 策略映射 A
• 使用 @SecondaryTable 映射 BB、CC 等注解。

我没有测试过这个，我不知道它是否适用于 BB1、BB2。

引用

• Java 与 Hibernate 的持久性
  • 5.1.5 混合继承策略 (p207-p210)