Inheritance and subclasses fetching

Mykola · **Joined:** Fri Mar 16, 2007 7:40 am **Posts:** 4

Hi,

I have a problem with Inheritance and subclasses fetching. Basically I have very simple inheritance structure like the following:

Code:

@Entity
public class Base {
}

@Entity
public class Child extends Base {
}

The problem is that when I'm querying for a Base class, it returns all subclasses as well. But I need only a Base class instances. I tried to use different inheritance strategies, and discriminators, but it didnt' help.

Finally I thought I found a sollution, joined inheritance and fetch='select', but it was for xml mapping.

And I don't know, and can't find anywhere how to set fetching mode (fetch=select) for an Entity using annotations. Is it possible at all?

thanks!

scesbron · **Joined:** Thu Sep 04, 2003 8:23 am **Posts:** 368

In hbm files you can also set polymorphism="explicit" but I don't know if this is available through annotations

Mykola · **Joined:** Fri Mar 16, 2007 7:40 am **Posts:** 4

scesbron wrote:

In hbm files you can also set polymorphism="explicit" but I don't know if this is available through annotations

Yes, it is available. I tried to set it to explicit, but again got the same behaviour :(

Here is the exact code I'm using:

Code:

@Entity()
@org.hibernate.annotations.Entity(polymorphism = PolymorphismType.EXPLICIT)
@Inheritance(strategy=InheritanceType.JOINED)
public class Base {
}

@Entity
@PrimaryKeyJoinColumn(name="id", referencedColumnName = "id")
public class Child extends Base {
}

As I understand, but correct me if I'm wrong, for some reason, when there is a query for a superclass, it doesn't check whereas there is a record in the joined table for a subclass.

frusso · **Posted:** Fri Mar 16, 2007 12:23 pm

Would you consider changing your own class hierarchy? Perhaps

Code:

@Entity
public abstract class AbstractBase {
}

@Entity
public class Base extends AbstractBase {
}

@Entity
public class Child extends AbstractBase {
}

If you query for Base, you get only Base, but if you query for AbstractBase you get both. I would suggest that if you want queries for Base not to include instances of Child, then Child isn't really a true subclass of Base in the first place.

Mykola · **Joined:** Fri Mar 16, 2007 7:40 am **Posts:** 4

Actually, I can't change the class hierarchy. The thing is that the base class is a part of the existing application. And in that application there are a lot of queries for a base.

Now I need to add a child of the base, but all that queries should fetch the base only.

scesbron · **Joined:** Thu Sep 04, 2003 8:23 am **Posts:** 368

So why do you want to set inheritance in hbm files ?
You can have inheritance in java but not specify it in hibernate. Thus this classes are not correlated

Mykola · **Joined:** Fri Mar 16, 2007 7:40 am **Posts:** 4

scesbron wrote:

So why do you want to set inheritance in hbm files ?
You can have inheritance in java but not specify it in hibernate. Thus this classes are not correlated

But these classes are correlated. Child class should have something from the Base class, also it has to introduce something own. And Child class should behave like a base in some cases.
Furthermore they have to share at least the id column to save the existing relationships of the Base class with other classes.

I figured out that when hibernate querying for Base class, it usesleft outer join to the Child class. Thats why it fetches all records from the Base table.
And I need to specify somehow that when querying for a Base class, it needs to check the Child table. Or it should be specified for a Child class, not to fetch it, until the explicit query will ask for it.

But I don't know how to specify such kind of limitation.