将超类和子类映射到Hibernate中的不同表

Question

I want to map a class to a table and a subclass to another table. I'm not sure if you can do this in a proper way. I tried this so far:

@javax.persistence.Entity
@Inheritance(strategy = InheritanceType.JOINED)
@Table(name = "MyTable1")
public class MyClass

@javax.persistence.Entity
@Inheritance(strategy = InheritanceType.JOINED)
@Table(name = "MyTable2")
public class MySubclass extends MyClass

It works but in the generated SQL both tables are joined and I want to keep them seperated.

Another approach was to use the same as above but change JOINED to TABLE_PER_CLASS. This does not work because there is a UNION ALL on both tables in the SQL when I query MyClass .

I also tried to use @org.hibernate.annotations.Entity(polymorphism = PolymorphismType.EXPLICIT) and @org.hibernate.annotations.Entity(polymorphism = PolymorphismType.IMPLICIT) but with no effect.

So my question is if there is a way to map this in Hibernate. Any suggestions?

Thanks!

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Using Hibernate 3.6.1

Answer 1

It seems you want to consider the two entities as completely unrelated, except for the fields they have in common. It's not really an inheritance relationship since when you search for a MyClass, you don't want MySubclass instances to be found, although MySubClass instances are instances of MyClass in your design.

The best solution is thus probably to use a MappedSuperclass (see chapter 2.2.4.4) which contains the common fields and methods, and to create two completely separate entities which extend this mapped super class.

Answer 2

TABLE_PER_CLASS should do what you want.

Note that when you execute a query like from MyClass m , Hibernate loads all objects of class MyClass , including subclasses, hence the UNION ALL clause. If you need only MyClass without subclasses, you need to use the following query: from MyClass m where m.class = MyClass .