Question

Comics object can have many Chapter object.

I have this in Comics class:

@OneToMany(targetEntity=Chapter.class, mappedBy="comics", fetch=FetchType.LAZY, cascade={CascadeType.PERSIST, CascadeType.REMOVE})
private List<Chapter> chapters = null;

My method in adding a chapter to a Comics:

public Chapter addChapter(Chapter chapter, String key) {
    EntityManager em = EMF.get().createEntityManager();
    EntityTransaction tx = null;
    Comics comics = null;
    try{
        tx = em.getTransaction();
        tx.begin();

        comics = em.find(Comics.class, KeyFactory.stringToKey(key));
        chapter.setPages( new LinkedList<Page>() );

        comics.getChapters().add(chapter);

        tx.commit();
    }catch(Exception ex){
        ex.printStackTrace();
        if(tx != null && tx.isActive())
            tx.rollback();
    } finally{
        em.close();
    }

    return chapter;
}

My method for reading a Comics:

public Comics read(String key) throws IllegalAccessException, InvocationTargetException{
    EntityManager em = EMF.get().createEntityManager();
    Comics comics = new Comics();
    try{
        Comics emComics = em.find(Comics.class, KeyFactory.stringToKey(key));
        BeanUtils.copyProperties(comics, emComics);
        comics.setChapters(new LinkedList<Chapter> (emComics.getChapters()));

    }finally{
        em.close();
    }
    return comics;
}

When I saved new Comics, I also have:

comics.setChapters( new LinkedList<Chapter>() );

The problem is that read method return unexpected ordering of chapters. What would be the best approach to display chapters in order?

Answer 1

You can use @OrderBy annotation:

For example:

@OneToMany(targetEntity=Chapter.class, mappedBy="comics", fetch=FetchType.LAZY, cascade={CascadeType.PERSIST, CascadeType.REMOVE})
@OrderBy("chapterNumber")
private List<Chapter> chapters = null;

This is assuming there is a chapterNumber field which is comparable