Find and delete an item from a collection - jpa

Question

I have the following problem with a method of JPA.
I can not delete an item from a collection.
Actually, the method only works if it is not the last element inserted.
Where I'm wrong?
This is the class of my model:
#Entity
public class JobOffer {
#SequenceGenerator(name="JobOffer_Gen", sequenceName="JobOffer_Seq", initialValue=1, allocationSize=1)
#Id #GeneratedValue(generator="JobOffer_Gen") #Column(name="ID_JOBOFFER")
private Long id;
#Column
private String title;
...
#OneToMany
#JoinTable(joinColumns = #JoinColumn(name = "JOBOFFER_IDFK"), inverseJoinColumns = #JoinColumn(name = "LANGUAGE_IDFK"))
private Collection<Language> languages = new HashSet<Language>();
...
}
This is my method in JPA:
#Override
#Transactional
public void deleteLanguage(JobOffer joboffer, Long idLingua) throws BusinessException {
for(Language l : joboffer.getLanguages()){
if (l.getId() == idLingua){
joboffer.getLanguages().remove(l);
}
}
em.merge(joboffer);
}
What is the correct way to search for an item in a collection and delete in JPA?
This is the error that I get from the console:
21-ott-2013 18.22.27 org.apache.catalina.core.StandardWrapperValve invoke
GRAVE: Servlet.service() for servlet [jobbook] in context with path [/jobbook] threw exception [Request processing failed; nested exception is java.util.ConcurrentModificationException] with root cause
java.util.ConcurrentModificationException
at java.util.AbstractList$Itr.checkForComodification(AbstractList.java:372)
at java.util.AbstractList$Itr.next(AbstractList.java:343)
at org.eclipse.persistence.indirection.IndirectList$1.next(IndirectList.java:571)
at it.univaq.mwt.j2ee.jobbook.business.impl.JPAJobOfferService.deleteLanguage(JPAJobOfferService.java:95)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:597)
and then continues

Answer 1

You may try something like this:
#Override
#Transactional
public void deleteLanguage(JobOffer joboffer, Long idLingua) throws BusinessException {
Language language = em.createQuery("SELECT lang from Language lang where lang.jobOffer.id = :job_id", Language.class).setParameter("job_id", joboffer.getId()).getSingleResult();
if(language != null){
joboffer.getLanguages().remove(language);
}
}
Please note:
this is not tested
I assumed that your "Language" Entity has a reference to your JobOffer Entity. Although I do not know your domain, this may indicate a bad design
If the parameter joboffer is in detached state, then you have to reattach it to the current session (using em.merge()).
Edit: Adapted Query in response to the comment of DataNucleus.