Updating collection of dependent objects

I have a class Event and a dependent class Entry, which instances are only valid in the context of an event.

What would be the best way to model this in JDO? Acutally I don't want to query for Entries only for Events and their entries. So do I need a key on Entry?

My current solution is:

@PersistenceCapable
public class Event {
    @PrimaryKey
    @Persistent(valueStrategy = IdGeneratorStrategy.IDENTITY)
    public Long id;

    @Persistent
    public List<Entry> entries = new ArrayList<Entry>();
}

@PersistenceCapable
public class Entry {
    @PrimaryKey
    @Persistent(valueStrategy = IdGeneratorStrategy.IDENTITY)
    public Key key;

    @Persistent
    public String name;
}

I tried to add an entry to an existing Event, but it doesn't acutally persist the changed Event:

Event e = null;
PersistenceManager pm = PMF.get().getPersistenceManager();
try {
    e = pm.getObjectById(Event.class, Long.parseLong(id));

    System.out.println(e.entries.size());

    Entry entry = new Entry();
    entry.name = name开发者_运维技巧;

    e.entries.add(entry);

    pm.makePersistent(e);

    System.out.println(e.entries.size());
} catch (NumberFormatException nfe) {
    return null;
} finally {
    pm.close();
}
return e;

I tried to make Entry an Embedded entity, but it is not allowed to have collections of embedded objects.

Actually updating was not the problem. I did not load the event properly in my service.

public Event loadEvent(String id) {
    PersistenceManager pm = PMF.get().getPersistenceManager();
    try {
        Event event = pm.getObjectById(Event.class, Long.parseLong(id));
        // And load entries
        for (Entry entry : event.getEntries()) {
            entry.amounts.size();
        }
        return event;
    } catch (NumberFormatException e) {
        return null;
    } finally {
        pm.close();
    }
}

After adding adding the lines between the comment and the return statement all entries were shown properly.