Hibernate: No session after save of new entity

Question

I create a new entity, store it the first time and then want to access the collections of the related classes:

 @Override
 protected void onSubmit(final AjaxRequestTarget target, final Form<?> form) {           
        final E entity = (E) form.getModelObject();
        getDao().save(entity); //calls session.saveOrUpdate(entity)
        LOG.debug("Saved entity " + entity);
        LOG.debug("Show collections " + entity.getField().getListOfSomething());
        parent.replaceContentPanel(parent.getDetailsPanel(parent.createReloadableModel(entity)), target);
 }

I get the following error on the second line of logging:

org.hibernate.LazyInitializationException: 
failed to lazily initialize a collection of role: 
no session or session was closed

I have also tried

Hibernate.initialize(getDetailsModel().getObject().getField().getListOfSomething());

This leads to a different error:

org.hibernate.HibernateException: collection is not associated with any session

This is not very surprising when debugging I can see that the collection proxies have no session associated with them.

I am using the 'openSessionInView' filter that comes with the Spring framework. The code works fine by the way when I want to update an existing entity. It also works when I set the fetchType to eager on the collection:

@OneToMany(mappedBy = "field", fetch = FetchType.EAGER)
private List<E> listOfSomething= new ArrayList<E>();

Do I really need to set this to EAGER? I want to avoid this very much and was hoping for a way around it. Is there a way to associate a newly stored entity with the Hibernate session? I have tried both a session.load(entity) and a session.merge(entity) with no success.

My entities look like this:

@Entity class A { 
    @ManyToOne B b;
}

@Entity class B {     
   @OneToMany(mappedBy = "b") List<A> aList;
   @OneToMany(mappedBy = "b") List<C> cList;
}

@Entity class C {
   @ManyToOne B b;
}

What I am doing is creating a c = new C(), selecting b from a DropDownChoice and on submit I want to persist getDao().save(c). After the submit I want to display my new entity on a different panel which is loaded via ajax. For example I want to display c.getB().getAList() and that is where the exception happens because the bI selected from the DropdownChoiceis a detached entity and it's aList was not fetched and cannot be loaded lazily either.

I cannot figure out how to get the still existing session into my new C instance.

Here's more info from my web.xml

<filter>
    <filter-name>opensessioninview</filter-name>
    <filter-class>org.springframework.orm.hibernate3.support.OpenSessionInViewFilter</filter-class>        
</filter>
<filter-mapping>
    <filter-name>opensessioninview</filter-name>
    <url-pattern>/*</url-pattern>
</filter-mapping>

Answer 1

An appropriate workaround is to merge() the object that contains the collection before accessing the collection.

Answer 2

I found a work-around that allows me to enforce loading the collections I know I will need with Hibernate.initialize(...)

final ListModel<B> listModel = new ListModel<B>(bList);
for (final B b : bList) {
   Hibernate.initialize(b.getAList());
}
add(new DropDownChoice<B>("b", listModel, new ChoiceRenderer<B>("name", "id")));

When I add lines 2-4 from this snippet to my form all instances of B that I might later select for my new instance of C will have an initialized list of A.

I am only half happy with this. I'd still prefer to be able to attach an object to the hibernate session on demand.