As a side note...
The LazyInitialization exception wasn't an issue for me when using the
"OpenSessionInView" pattern. However it became a large issue when trying
to use hibernate to back a thick-client GUI. With what I've used of lift,
I can't see JPA being any more difficult to manage then using another
web-framework (perhaps even simpler in some regard). The key is just to
understand what leaving the hibernate session open does to your transaction
length and how you cna remedy troubles as you see them.
-Josh
On Sat, Dec 6, 2008 at 9:59 PM, philip <[EMAIL PROTECTED]> wrote:
>
>
> Hi Derek,
>
> Thanks, these are good reasons to use the JPA.
>
> About the use of the JPA in SEAM - the SEAM in Action (http://
> manning.com/dallen/) book says
> "Lazy loading of entity associations has unfortunately established a
> bad reputation. The first thing that comes to mind in most developers'
> minds when you talk about lazy loading is Hibernate's
> LazyInitializationException. The culprit is the detached entity
> instance.
> ...
> If the persistence manager is closed after the action method is
> invoked, the Course instance is detached when the view is rendered.
> ...
> A call to the method getHoles() triggers an exception because the
> EntityManager that loaded the Course instance is no longer available
> to further communicate with the database. The same problem arises on
> postback, even if a lazy association wasn't hit in the view."
>
> So within Liftweb it would then make sense to place the JPA
> persistence manager within a stateful snippet. Since the snippet is
> bound to the session, then navigation of the lazy loading entitys
> wouldn't cause a LazyInitializationException?
> Is this what is happening on the JPA example within Liftweb? it
> doesn't look like it - in file Books.scala
> def add (xhtml : NodeSeq) : NodeSeq = {
> def doAdd () = {
> Model.merge(book)
> redirectTo("list.html")
> }
>
> The SEAM book says
> "Merging is a crude operation and should be avoided if possible. It
> first loads an entity
> instance with the same identifier as the detached instance into the
> current persistence
> context, resulting in a database read. Then, the property values from
> the detached
> instance are copied onto the properties of the managed instance. The
> main problem
> with this operation is that merging clobbers any changes that may have
> been made to
> the database record since the detached instance was retrieved (unless
> object version-
> ing is used). There are other problems as well. If an entity instance
> with the same
> identifier as the detached instance has already been loaded into the
> current persis-
> tence context, a non-unique object exception is thrown because the
> uniqueness con-
> tract of entities in a persistence context is violated. You may also
> run into a lazy
> loading exception if you hit an uninitialized association on the
> detached instance dur-
> ing the merge. Avoid merging if at all possible. "
>
> Thanks, Philip
>
>
>
>
>
> On Dec 6, 10:47 pm, "Derek Chen-Becker" <[EMAIL PROTECTED]> wrote:
> > Here are a couple of features that JPA has that can be very useful:
> >
> > 1. More powerful query language. The tradeoff is that HQL is not
> > type-safe like Mapper's findXXX methods, so you need to test your
> query
> > syntax to make sure it returns what you want. Granted, you can use SQL
> > directly in Mapper, but it's tied directly to the DB while HQL allows
> you to
> > abstract away some of the smaller details, particularly with join
> > associations. However, HQL does allow you to easily
> > 1. Do flexible joins between large graphs of objects
> > 2. Select scalar values (i.e. "select distinct player.age from
> > TeamMember player where ...")
> > 3. Prefetch otherwise lazily loaded properties ("select player from
> > TeamMember left join fetch player.coach")
> > 2. IMHO, better definition of associations between classes.
> > HasManyThrough works well for a subset of usages, but it:
> > 1. Can't be treated as a real, live collection (no updates, no
> > cascades)
> > 2. Only runs once (it's lazy). Given the duration of a web request
> > cycle this would usually not be a big deal unless you have a
> > high number of
> > concurrent users operating on the same data
> > 3. Doesn't map well to many-to-many relationships; you have to
> build
> > your own binding entity to represent the join table
> > 3. JPA has a number of implementations available for an intermediate
> > cache to improve performance
> > 4. JPA supports locking of objects for read and write
> >
> > That's what I can come up with off the top of my head. Having SEAM
> generate
> > the classes for you is also nice, although I'm sure a similar tool could
> be
> > written for Mapper. Now, as for SEAM's (ab)use of the entity manager (I'm
> > assuming that's what you mean by transaction manager), I'm not so sure
> that
> > it's actually keeping the EM open across the entire "conversation", but
> > rather providing an interface that makes it appear that it is. I saw
> Gavin
> > King here give a presentation on SEAM a few years ago and it sounded like
> > they were doing some magic behind the scenes to keep everything coherent.
> I
> > don't really know enough about it to argue for or against it, but
> generally
> > the idea is that by holding a transaction across the scope of an entire
> > conversation you can treat a series of forms/pages as an atomic unit of
> > work. I'm pretty sure Mapper doesn't have anything comparable, and
> Mapper's
> > concept of lazy loading isn't tied to having an open EM like it is in
> JPA.
> > That means that if something in Mapper is going to be lazily loaded,
> it'll
> > work no matter what.
> >
> > Derek
> >
> >
> >
> > On Fri, Dec 5, 2008 at 6:24 PM, philip <[EMAIL PROTECTED]> wrote:
> >
> > > Hi,
> >
> > > Why should I use JPA as vs using Liftwebs built in database mappers.
> > > Lets say I build a 100 table mysql database, I'm mostly doing forms,
> > > user input, tables out.
> > > Is there something unattractive about using Liftwebs built in mapper?
> > > I don't like the name MetaMegaProtoUser, but thats just aesthetics.
> >
> > > Currently I can generate lots of Java JPA classes from database using
> > > SEAMs generation ant script, my thought was then to use those Java
> > > classes and convert them over to Scala JPA as in the example project.
> > > However I don't know why to do this as opposed to using Liftweb's
> > > mapper.
> >
> > > I'm also worried about something and not sure if I should be worried
> > > about it. JBoss SEAM has this concept of conversational state and it
> > > can keep the JPA transaction manager open from one page to another
> > > page, through use of the session. SEAM claims this is a good idea and
> > > we shouldn't be closing the transaction manager on each call. Also it
> > > can keep a transaction open for a long time through page views.
> > > However, I don't see why or when I would need to do a transaction with
> > > multiple page views involved in it. I mean, even large websites like
> > > facebook, I doubt they do this - they use PHP. Why not just do the
> > > transaction in one call?
> >
> > > SEAM also has this concept of avoiding the Hibernate
> > > LazyInitializationException, so if your navigating down some objects
> > > relations, it will load the lazy relations as you navigate down. So
> > > you don't need to re-query the database, because of the transaction
> > > manager being open between pages, it will load the Hibernate objects
> > > that are lazy loaded as you navigate a O-R structure.
> > > Of course this sounds attractive in regards to SEAM.
> > > What if I navigated structures in Liftweb using the built in mapper -
> > > is there any concern I should have?
> >
> > > Thanks, Philip
>
> >
>
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