NHibernate : pattern for returning fully loaded instances from repositories

Question

Hi all

As part of my endless NHibernate-inspired DAL refactoring purgatory, I have started to use the Repository pattern to keep NHibernate at arms length from my UI layer. Here's an example of a Load method from a repository.

public StoredWill Load(int id)
{
  StoredWill storedWill;
  using (ISession session = NHibernateSessionFactory.OpenSession())
  {
    storedWill = session.Load<StoredWill>(id);
  }
  return storedWill;
}

I love the fact that my website doesn't know what an ISession is.

Naturally, I started getting lazy initialisation exceptions because the method above doesn't load the StoredWill, it just returns a proxy. When you access the proxy's properties, you get the exception because you are ro longer within the scope of the ISession. I laughed out loud when I realised what was happening.

I have fixed this with:

public StoredWill Load(int id)
{
  StoredWill storedWill;
  using (ISession session = NHibernateSessionFactory.OpenSession())
  {
    storedWill = session.Load<StoredWill>(id);
    string iReallyCouldntCareLess = storedWill.TestatorLastName;
  }
  return storedWill;
}

But it all seems a little silly. Does anyone use a slightly more elegant pattern?

Love you guys.

David

Answer 1

Check out the ncommon framework; it has a nice implementation of fetching strategies that are perfect for this sort of task. You can read more about how this is implemented on the author's blog.

I should also note that your session usage is a little off...you should really be controlling the lifecycle of your session higher up (say at the controller or httpmodule level depending on your front-end). Opening a new session for each repository action is a major anti-pattern in the nhibernate world.

Answer 2

public StoredWill Load(int id)
{
  StoredWill storedWill;
  using (ISession session = NHibernateSessionFactory.OpenSession())
  {
    storedWill = session.Load<StoredWill>(id);
    // force an eager load within the session
    NHibernateUtil.Initialize(storedWill.TestatorLastName);
  }
  return storedWill;
}

Answer 3

Use dependency injection and pass an ISession to your repository classes through their constructors. That's the only way to allow multiple repositories to participate in the same transaction.

Your website should have some knowledge of an ISession because that's where the transaction boundaries are defined. You can use session-per-request pattern so that the ISession is only referenced in an HttpRequest module or Global.asax. Or you can use a framework or embrace NHibernate and control transactions on the pages.