Question

what is the best approach to load a collection and update all items with NHibernate. The current code loads 50 objects and processes each in its own transaction (if 1 fail others are OK).

NH Profiler says that there are too many sql queries per session.

After all, what do you think about this code?

using (var session = sessionFactory.OpenSession())
        {
            var myCollection =
                (from obj in session.Query<MyObject>()
                 select obj).Take(50);

            foreach (var item in myCollection)
            {
                using (var tx = session.BeginTransaction())
                {
                    try
                    {
                        //  Do some stuff...
                        session.Update(item);
                        tx.Commit();
                    }
                    catch (Exception)
                    {
                        tx.Rollback();
                    }
                }
            }
        }

Answer 1

From NHibernate: Streaming large result sets :

NHibernate is meant to be used in an
OLTP system, as such, it is usually
used in cases where we want to load a
relatively small amount of data from
the database, work with it and save it
back. For reporting scenarios, there
are better alternatives, usually (and
before you ask, any reporting package
will do. Right tool for the job, etc).

But there are cases where you want to
do use NHibernate in reporting
scenarios nonetheless. Maybe because
the reporting requirements aren’t
enough to justify going to a separate
tool, or because you want to use what
you already know. It is in those cases
where you tend to run into problems,
because you violate the assumptions
that were made while building
NHibernate.

using (IStatelessSession s = sessionFactory.OpenStatelessSession())
{
    var books = new ActionableList<Book>(book => Console.WriteLine(book.Name));
    s.CreateQuery("from Book")
        .List(books);

}

The stateless session, unlike the
normal NHibernate session, doesn’t
keep track of loaded objects, so the
code here and the data reader code are
essentially the same thing.

Basically, use a stateless session and batching. Read also: NHibernate Perf Tricks