@PersistenceCapable(detachable = "true")
public class Item {
    ...
    @Persistent(defaultFetchGroup = "true")
    Collection<Subitem> subitems;
    ...
}

@PersistenceCapable(detachable = "true")
public class Item {
    ...
    @Persistent(defaultFetchGroup = "true")
    @Element(column="itemId")
    Collection<Subitem> subitems;
    ...
}