problem with inheritance and entities

dmurguia · **Posted:** Fri May 16, 2008 12:23 am

I am using Hibernate Annotations 3.2.1 and Hibernate 3.2.4 from Jboss Seam 2.0.0.

I have four entity classes, lets call them A, AA, B and BB with the following relationships:

- AA extends A
- BB extends B
- A has a one to many relationship with B.

Now I want to add a restriction: If the entity class is A, it should have one to many relationship only with entities of class B; conversely, If the entity class is AA, it should have one to many relationship only with entities of class BB.

My first approximation was to create two generic mapped superclasses A0 and B0 and use them as a base this way:

Code:

@MapperSuperclass
class A0<X> {
...
   @OneToMany
   Set<X> getChildren() {
...
}

@MappedSuperclass
class B0<X> {
...
   @ManyToOne
   X getParent() {
...
}

@Entity
class A extends A0<B> {
...
}

@Entity
Class B extends B0<A> {
...
}

@Entity
class AA extends A0<BB> {
...
}

@Entity
Class BB extends B0<AA> {
...
}

but I am not happy with result because I see this as too convoluted and I lose the inheritance between (A and AA) and (B and BB).

How can I implement this structure?

sanne.grinovero · **Posted:** Fri May 16, 2008 6:46 am

Hi,
Having to override a relationship is not very clean IMHO.
if you think about the physical schema you are going to generate you see there is much unnecessary complexity.

I would advise you to NOT have AA extend A, if they have all other code in common they could both extend a same non-entity class. The same for B classes.
So you don't have to share the same table for parent and childs, this sharing would make it impossible to enforce your requirement at a DB level.

A completely different approach is you enforce you requirement at Business code level, not very nice.

regards,

dmurguia · **Posted:** Fri May 16, 2008 12:16 pm

Hi,

Thank you for your post. I should have said that I already have a database structure like this:

Code:

create table A(
   column id integer,
   column colA varchar(10),
   primary key(id)
);

create table AA(
   column id integer,
   column colAA varchar(10),
   primary key(id),
   foreign key(id) references A
);

create table B(
   column id integer,
   column colB varchar(10),
   column idA integer,
   primary key(id),
   foreign key idA references A
);

create table BB(
   column id integer,
   column colBB varchar(10),
   primary key(id),
   foreign key(id) references B
);

Because of restrictions of my project, you can regard this database schema as unchangeable.

As you can see, I can't enforce my restriction at the DB level unless I write some combination of triggers and stored procedures that I haven't analyzed yet. I still believe I can enforce the restriction at the Entity Bean level, tough.

sanne.grinovero · **Posted:** Fri May 16, 2008 12:55 pm

Hi,
having seen your schema I would really suggest you to make 4 different entities,
than maybe use interfaces to define common types.