Updating a reference to a FK - jpa

Question

I've searched for quite a while trying to figure this out. I am using JPA with EclipseLink (Oracle DB). I have a lookup table full of values. I have another table that has a FK relationship to that table. I can insert data fine, but when I try to update the table with a different value, I get an exception. I've tried setting the CASCADE_TYPE but that doesn't have any impact. I thought this would be simple, but maybe I'm missing something.
Lookup table:
public class SomeType implements Serializable {
#Id
#Column(name = "ID")
private Short id;
#Column(name = "TYPE")
private String type;
:
(getters & setters)
}
Contents:
ID Type
------------
1 Type1
2 Type2
3 Type3
: :
Person table (I've left out the Sequencing stuff for brevity):
public class Person implements Serializable {
#Id
#Column(name = "ID")
private Short id;
#JoinColumn(name = "SOME_TYPE", referencedColumnName = "ID")
#ManyToOne(optional = false)
private SomeType someType;
:
(getters & setters)
}
Inserting works fine:
EntityManager em;
:
Person p = new Person();
p.setSomeType(new SomeType(1));
em.persist(p);
That results in:
ID SOME_TYPE
------------------
1 1
But if I want to update Person to change the type:
EntityManager em;
:
Person p = em.find(1);
SomeType newtype = new SomeType(2);
p.setSomeType(newtype);
em.merge(p);
I see the following exception:
Exception [EclipseLink-7251] (Eclipse Persistence Services - 2.5.2.v20140319-9ad6abd): org.eclipse.persistence.exceptions.ValidationException
Exception Description: The attribute [id] of class [SomeType] is mapped to a primary key column in the database. Updates are not allowed.
All I want is the value in the Person table to be updated, like:
UPDATE PERSON set SOME_TYPE = 2 where ID = 1;
Any assistance would be greatly appreciated. Thanks.

Answer 1

Thanks to Chris for answering this:
The update is possible if you use a reference to the managed instance of the object that you want to refer to, not create a new instance.
Person p = em.find(1);
p.setSomeType(em.find(SomeType.class, 2));
em.merge(p);