Dynamically modifying mapped instance class to superclass

mkadan · **Joined:** Fri Aug 15, 2008 10:31 am **Posts:** 2

Let's say I have three classes:

class A { ... }
class B extends A { ... }
class C extends A { ... }

They are mapped to DB using Hibernate's JOINED strategy.

B myB = new B();

When I persist myB, I have A's attributes placed into table "A", and B's attributes placed into table "B".

Now I want to change myB class to A, which actually means I want to clean table "B" of it's datarow (and all datarows in tables that FK-ed on table B). I cannot just remove this instance from DB and then put a new instance of class A, because I also have lots of other tables with FK restrictions on table "A", and I don't want this restrictions to be broken or cascaded some way.

So, what I need is:

1) persist "myB", now I have a new datarow in table "A" with PK; and a new datarow in table "B", which has it's own PK and also a FK to "A"
2) I fill some other tables which may have FK to A's or to B's PK
3) I want to change "myB" class to A, so I need datarow in table "B" deleted, along with all datarows that FK-ed to it (that means which FK-ed to B's PK). And *IT'S IMPORTANT*, that A's datarow and all datarows in other tables that FK-ed on A's PK must remain!
4) I want to change "myB" class to C, so I need to insert datarow in table "C" with new PK and a FK on A's PK

How could this be accomplished?

schauder · **Joined:** Thu Jul 05, 2007 9:38 am **Posts:** 287

This really sound like design mistake, because an Object should never change it's class.

I don't know what the real world reason for this desire is, but I try to give some ideas for a potential solution anyway.

How about this setup

Code:

class A{
Set<Role> roles;// a Map might be better here, or  a single instance

public <R extends Role> R getRole(Class<R> roleClass){
...
 }
}

@inheritance(TablePerConcreteClass)
abstract class Role{
A theA
....}

class B extends Role{...}

class C extends Role{...}

This way every A can provide a B or C orboth, and everything can be mapped with standard Hibernate mappings.

mkadan · **Joined:** Fri Aug 15, 2008 10:31 am **Posts:** 2

Thanks, your idea was just what I needed.