Mapping a hierarchy as "disjoint" entities

memeplex · **Joined:** Wed Apr 18, 2007 1:44 pm **Posts:** 27

Hi all,

I have a hierarchy where class B extends class A. Both are entities:

Code:

@Entity
class A {
}

@Entity
class B extends A{
}

with the above annotations hibernate will try to map A and B as a SINGLE_TABLE hierarchy.Also, with TABLE_PER_CLASS it will forbid AUTO ids. But as I don't care to treat A and B polymorphically it would be fine for me to just map them as different classes, disregarding their inheritance relationship. Something like that could be achieved, although a bit laboriously, as:

Code:

@MappedSuperclass
abstract class  X {
}

@Entity
class A extends X{
}

@Entity
class B extends X {
}

Is there any way to do the same thing avoiding this somewhat synthetic abstract class expedient.

Thank you in advance
Cheers,
Carlos

emmanuel · **Posted:** Wed Jun 20, 2007 8:18 pm

I have hard time to believe that A and B are stored in the same table (SINGLE_TABLE). You might have make a mistake somewhere.

memeplex · **Joined:** Wed Apr 18, 2007 1:44 pm **Posts:** 27

emmanuel wrote:

I have hard time to believe that A and B are stored in the same table
(SINGLE_TABLE). You might have make a mistake somewhere.

Believe it or not here is the schema autogenerated for a couple of simple classes in the inheritance relationship described in my previous post. Note that fields of both classes are mapped to the same table. Also, the other inheritance mappings are not an option, I don't want inheritance from hibernate point of view at all. I got that with the abstract @MappedSuperclass workaround, but I think there should be an easier way.

Code:

@Entity
public class A {
    
    @Id
    @GeneratedValue(strategy=GenerationType.AUTO)
    private Integer id;
    private Integer fieldOfA;

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }    
}
@Entity
public class B extends A {

    private Integer fieldOfB;
}

Code:

CREATE MEMORY TABLE A(

DTYPE VARCHAR(31) NOT NULL,

ID INTEGER GENERATED BY DEFAULT  AS IDENTITY(START WITH 1) NOT NULL PRIMARY KEY,

FIELDOFA INTEGER,

FIELDOFB INTEGE R)

Cheers,
Carlos

emmanuel · **Posted:** Wed Jun 20, 2007 10:24 pm

That's different than what you described. In you last example B extends A

memeplex · **Joined:** Wed Apr 18, 2007 1:44 pm **Posts:** 27

emmanuel wrote:

That's different than what you described. In you last example B extends A

You're not getting my point I guess. Originally I had:

Code:

@Entity
class A {
  // class A stuff
}

@Entity 
class B {
  // class B stuff
}

But this mapped everything (both class A stuff and class B stuff) to the same table. I could use joined tables or table per class instead, but anyway I don't care about mapping the hierarchy, just about reusing A stuff inside B at java level. And not without reasons, suppose I map the hierarchy as:

1) single table per hierarchy: I don't want a table mixing A stuff and B stuff
2) table per class: that's fine, but I can't autogenerate ids anymore.
3) joined tables: don't want the additional join

So, not finding another way, I refactor the above to:

Code:

@MappedSuperclass
abstract class X {
  // class A stuff
}

@Entity class A extends X {
}

@Entity class B extends X {
  // class B stuff
}

Now I reutilize class A stuff and both A and B are mapped to different classes without any id generation restriction.

Cheers,
Carlos

emmanuel · **Posted:** Thu Jun 21, 2007 3:13 pm

ah right i read too fast :)