Linking two object by code (not ID) using Eclipselink JPA - jpa

Question

i have two tables:
area (
id int PK autoincrement
code varchar
)
products (
id int PK autoincrement
area_id int
)
And the objets are defined like this:
class Product {
...
#JoinColumn(name = "area_id", referencedColumnName = "id")
#ManyToOne
#Expose
private Area area;
...
}
This works fine but I want that area to be a String with the code used in the table area column code.
class Product {
...
???
private String area;
...
}
What should be the annotations to make this work?
Thanks!

Answer 1

Try to use a combination of #SecondaryTable and #Column annotations. Something like this:
#Entity
#SecondaryTable(name="area", pkJoinColumns=#PrimaryKeyJoinColumn(name="id", referencedColumnName="area_id"))
class Product {
...
#Column(name="code", table = "area")
private String code;
...
}

Answer 2

If there is some poor soul with the same problem, here is how I did it:
Using transformers. So the field area is defined like this:
#Transformation(fetch = FetchType.EAGER, optional = false)
#ReadTransformer(transformerClass = AreaAttributeTransformer.class)
#WriteTransformers({
#WriteTransformer(
transformerClass = AreaFieldTransformer.class,
column = #Column(name = "area_id", nullable = false))
})
#Expose
private String area;
Then those clases work like this:
AreaAttributeTransformer
public class AreaAttributeTransformer implements AttributeTransformer {
private AbstractTransformationMapping mapping;
#Override
public void initialize(AbstractTransformationMapping abstractTransformationMapping) {
this.mapping = abstractTransformationMapping;
}
#Override
public Object buildAttributeValue(Record record, Object o, Session session) {
for (DatabaseField field : mapping.getFields()) {
if (field.getName().contains("area_id")) {
EntityManager em = MyEntityManagerFactory.getENTITY_MANAGER_FACTORY().createEntityManager();
List results = em.createNamedQuery("Areas.findById")
.setParameter("id", record.get(field))
.getResultList();
if (results.size() > 0)
return ((Area) results.get(0)).getCode();
}
}
return null;
}
}
AreaFieldTransformer
public class AreaFieldTransformer implements FieldTransformer {
private AbstractTransformationMapping mapping;
#Override
public void initialize(AbstractTransformationMapping abstractTransformationMapping) {
this.mapping = abstractTransformationMapping;
}
#Override
public Object buildFieldValue(Object o, String s, Session session) {
if (o instanceof RouSub) {
EntityManager em = MyEntityManagerFactory.getENTITY_MANAGER_FACTORY().createEntityManager();
List results = em.createNamedQuery("Area.findByCode")
.setParameter("area", ((Area) o).getCode())
.getResultList();
if (results.size() > 0)
return ((Area)results.get(0)).getId();
}
return null;
}
}