JPA: Error "entity is detached" when executing a query

Question

Hi experts,
I have two database tables with a foreign key constraint and generated JPA-entities for them:
@Entity
public class Verdeck implements Serializable {
     @EmbeddedId
     private VerdeckPK pk;
     @Column(name="ID_VERDECK")
     private String idVerdeck;
     @OneToMany(mappedBy="verdeck")
     @PersistenceContext
     private Set<Uzsb> uzsbCollection;
@Embeddable
public class UzsbPK implements Serializable {
     @Column(name="ID_UZSB")
     private String idUzsb;
     @Column(name="ID_PROJECT")
     private BigDecimal idProject;
Furthermore I have a SessionBean implementing a query in one of its business methods:
@WebMethod(operationName="getVerdeckData", exclude=false)
public List<Verdeck> getVerdeckData (@WebParam(name="searchkey")
     BigDecimal searchkey){
     Query q = em.createQuery("SELECT v FROM Verdeck v WHERE v.pk.idProject = :searchkey")
          .setParameter("searchkey", searchkey);
          return q.getResultList();
When calling the method via WebService-Navigator I get this error:
"com.sap.engine.services.webservices.espbase.server.additions.exceptions.ProcessException: The relationship >>uzsbCollection<< of entity (com.karmann.r57schraub.jpa.Verdeck(idProject=57, idIntern=v1))cannot be loaded because the entity is detached"
(idProject / idIntern) is the composed key of "Verdeck" and (57 / v1) is a concrete value for this key in the database table.
If required I could give you classes VerdeckPK and UzsbPK as well.
Could you please explain what I'm doing wrong?
Thanks for each hint,
Christoph

Answer 1

Hi Vladimir,
thank you for this hint! Especially for the article which provides the neccessary knowledge in background.
But using FetchType.EAGER does not solve my problem. I get a runtime error. In defaultTrace I get the following message
SAXException2: A cycle is detected in the object graph. This will cause infinitely deep XML: com.karmann.r57schraub.jpa.Verdeck@f9ac24 -> com.karmann.r57schraub.jpa.Uzsb@1f2a70 -> com.karmann.r57schraub.jpa.Verdeck@f9ac24]->com.sun.istack.SAXException2: A cycle is detected in the object graph. This will cause infinitely deep XML: com.karmann.r57schraub.jpa.Verdeck@f9ac24 -> com.karmann.r57schraub.jpa.Uzsb@1f2a70 -> com.karmann.r57schraub.jpa.Verdeck@f9ac24#
I already checked the records in my tables. There does not seem to be any cycle. Here's my test data:
VERDECK
ID_PROJECT    ID_INTERN     ID_VERDECK
57                    v1                  vext1
57                    v2                  vext2
UZSB
ID_PROJECT ID_UZSB ID_INTERN TYP_UZSB
57 ls1 v1 <null>
57 rs1 v1 <null>
57 sd1 v1 <null>
57 ls2 v2 <null>
57 rs2 v2 <null>
57 sd2 v2 <null>
Do you have any more idea?
Regards,
Christoph