Re: RequestFactory and (JPA) transaction concepts

On Thursday, November 1, 2012 6:36:54 PM UTC+1, Thomas Broyer wrote:

Guice Persist's @Transactional works great (of course with a ServiceLayerDecorator to instantiate your services through Guice): http://code.google.com/p/google-guice/wiki/Transactions

That's the missing guice-extension what I was looking for, thanks! (I'm already using Guice to inject a RequestScoped Provider<EntityManager> into ServiceLayerDecorator, Locator and ServiceLocator, and Guice Persist could help me to cut down some more boilerplate.)

 

I believe you MUST use a @RequestScoped EntityManager (again, Guice Persist FTW), to make sure you always have a single instance of a given (logical) entity. See http://code.google.com/p/google-web-toolkit/issues/detail?id=7341

Absolutely. (It's what I would expect anyway.)

And there are a few consequences that need consideration:

1.)

Since it's one and the same EntityManager, a call to persist() is actually only required for new entities. For existing entities, it can be enough to call 

  CatProxy editableProxy = ctx.edit(cat);

  editableCat.setName("newName");

  ctx.somethingThatSucceeds();

  ctx.fire();

Where the implementation of somethingThatSucceeds() can be empty! It's strange, that this call is necessary at all, but without it, SimpleRequestProcessor.processOperationMessages() determines, that operations == null, and returns immediately.

2.)

But there's also a danger involved in not calling persist(): With the following sequence of calls:

  CatProxy editableProxy = ctx.edit(cat);

  editableCat.setName("newName");

  ctx.somethingThatFails();

  ctx.somethingThatSucceeds();

  ctx.fire();

the entity will not be updated. This happens, because a rollback occurs in the first transaction, which detaches the object from the EntityManager. If we want it to be re-attached after that, we need to call merge().

So what I currently do, is to always call persist(), and to implement persist() like

  public void persist(final Cat cat) {

    if (cat.getId() == null)

      em.persist(cat);

    else {

      if (!em.contains(cat))

        em.merge(cat);

    }

  }

But this doesn't look perfectly good to me yet. How do you use and implement persist()?

--
You received this message because you are subscribed to the Google Groups "Google Web Toolkit" group.
To view this discussion on the web visit https://groups.google.com/d/msg/google-web-toolkit/-/hev_LM2qtO8J.
To post to this group, send email to google-web-toolkit@googlegroups.com.
To unsubscribe from this group, send email to google-web-toolkit+unsubscribe@googlegroups.com.
For more options, visit this group at http://groups.google.com/group/google-web-toolkit?hl=en.