@OrderBy被忽略

Question

I am trying to retrieve a ordered list of Elements from a OneToMany relation but it seems that @OrderBy is simply ignored?! The application uses eclipselink 2.1. The test data base is derby embedded.

Here are the relevant (imho) parts of the code:

@Entity
public class VotingEntity implements Voting {

    ...

    /**
     * A voting has at least one question.
     */
    @OneToMany(mappedBy = "voting", targetEntity = QuestionEntity.class, cascade = { CascadeType.PERSIST,
            CascadeType.REMOVE, CascadeType.REFRESH })
    @OrderBy("groupNumber")
    private List<QuestionEntity> questions = new ArrayList<>();

    ...

}

QuestionEntity:

@Entity
public class QuestionEntity implements Question {
    ... 
    /**
     * Could be used to group questions e.g. by pages.
     */
    private Integer groupNumber;
    ...
}

With this code i create a test entity:

public class VotingInstance {

    private VotingEntity voting;

    public VotingInstance() {
        voting = new VotingEntity();
        //other setters
        ...     
        voting.setQuestions(createQuestions(voting));
    }

    public VotingEntity getVoting(){
        return voting;
    }

    /**
     * 
     * @return a list with test questions.
     */
    private List<QuestionEntity> createQuestions(VotingEntity voting) {
        List<QuestionEntity> result = new ArrayList<>();
        //question 1
        QuestionEntity q1 = new QuestionEntity();
        q1.setVoting(voting);
        q1.setGroupNumber(3);
        ...
        result.add(q1);
        //question 2
        QuestionEntity q2 = new QuestionEntity();
        q2.setGroupNumber(1);;
        q2.setVoting(voting);
        ...
        result.add(q2);
                //question 3
        QuestionEntity q3 = new QuestionEntity();
        q3.setVoting(voting);
        q3.setGroupNumber(2);
        ...
        result.add(q3);
        return result;

    }

    ...

}

And here is the actual test:

public class PersistenceTest {

    @Rule
    public TemporaryFolder temp = new TemporaryFolder();

    EntityManager em;

    VotingEntity voting;

    @Before
    public void init() {
        voting = new VotingInstance().getVoting();
        em = new TestDbConfig(temp.getRoot().toString()).createEm();
        em.getTransaction().begin();
        em.persist(voting);
        em.getTransaction().commit();
        em.clear();
    }

    @After
    public void close() {
        em.close();
    }

    @Test
    public void checkVoting() {
        VotingEntity v = em.find(VotingEntity.class, voting.getID());
        for (Question q : v.getQuestions()) {
            System.out.println("q-id: " + q.getID() + " group: " + q.getGroupNumber());
        }
    }
}

The output is sorted by id (default without OrderBY) but q2 should be first (orderNumer=1 ) and q1 should be second?! So what to do to get an ordered list (i don't need to keep the order ind db, only the output is relevant).

UPDATE: Some comments suggest to add a third question and create output to check actual groupnumber. i tried this and all is fine. I updated the code above. The output is:

q-id: 2 group: 3
q-id: 6 group: 1
q-id: 8 group: 2

Answer 1

As per http://docs.oracle.com/javaee/5/api/javax/persistence/OrderBy.html , OrderBy only orders the QuestionEntity instances at the point the association is retrieved. The missing peice is that you seem to be creating the VotingEntity and are likely recreating the questions list at the same time - JPA will only set the order when it is returned from the database. It is up to the application to maintain it.

Try calling em.refresh(v) to see what I mean. This should cause the the collection to be refreshed from the database.

The problem you are encountering is that JPA allows for a second level cache. So when you create the VotingEntity with the unordered list, it sticks around unordered in the cache and isn't fetched from the database. To guarantee ordering, the application will need to maintain it at all times, or refresh it (and/or clear the cache) when order needs to be reasserted.