JPA Inheritance - Change the Entity type - jpa

Question

I have two classes
#Entity
#Inheritance(strategy=InheritanceType.SINGLE_TABLE)
#DiscriminatorColumn(name="PERSONTYPE")
#DiscriminatorValue(value="PERSON")
public class Parent {
.......
}
#Entity
#DiscriminatorValue(value="CHILD")
public class Child extends Parent{
.......
}
The scenario I have:
create a person -- then the PERSONTYPE = 'PERSON'
go the Person page and update it to be 'CHILD' by checking a check box 'Is Child' then after save the Person must be saved to be with type 'CHILD'.
Then how can I change the entity type from 'PERSON' to 'CHILD'?

Answer 1

Here are a couple possibilities:
It seems the obvious thing to do would be to setIsChild(true) on the Parent object and commit it, however I'm not sure how JPA will react to this since you are now committing a Parent and the result is a Child. Not sure this is possible. It is definitely worth a try.
Another option would be to write JPA update statement (circumventing Child and Parent objects) to update the is_child column in the database. Then when you subsequently query this record you will get a Child back not a Parent.
Lastly, you could create a child object with all values of the parent object, then delete the parent, and create the child. This will work, however aside from the extra processing required for delete / create, instead of a simple update, the id of the child may change (it will change if you are using auto generated ids). IMO, this is not a good solution, but it will work.

Answer 2

i assume you have unique properties for Child Object and thats why you want to use inheritance, otherwize just as #ZB Ziet commented you should just use child flag
Solution 1
i see you are using single table inheritance strategy, thus you have to modify the descriminator field manulay (by using SQL queries) and set appropirate fields on child table.
UPDATE Parent SET PERSONTYPE='CHILD' WHERE id = 1
practicaly you use native queries like this
enityManager.createNativeQuery(“UPDATE PERSON SET PERSONTYPE = ?, ”
“ VERSION = VERSION + 1 WHERE ID = ?”)
.setParameter(1, 'CHILD')
.setParameter(2,personID)
.executeUpdate();
then you can use entitymanager to get the child and set properties
entityManager.find(Child.class,1).childProp=xxxx
** Solution 2 **
IMO, The best thing to do here is instead of using single table strategy you should use joined table strategy
in joined table strategy new table entireis is created for each child having thier id as foreign key to parent entity.so changing the id will also set its parent
#Entity
#Inheritance(strategy = InheritanceType.JOINED)
public class Person {
...
it would have been cool if you can just create new child set the id same as parent and save. Child c = new Child(); c.setId(parent.getId()); entityManager.merge(c) . but hibernate tries to re-create a parent object resulting id confilict.
so the solution is to write native query
em.createNativeQuery("INSERT into child (id) VALUES (1)").executeUpdate(); //1 being the parent id
more reference on inheritance

Answer 3

Use EntityMaster
The Idea is to create an Entity just for changing the discriminator.
At least this Entity need to have the id and the discriminator.
#Entity
public class ParentMaster {
public static final PERSON="PERSON";
public static final CHILD="CHILD";
private Long id;
private String persontype;
//getter setter
}
Now load the ParentMaster by id and pm.setPersontype(ParentMaster.CHILD);.