Question

I’m using NH to map DB to an object named location which has some properties and on List <Attachments> attachments (one-to-many relation with instance of lcocation).

I use lambda expression to init location istance:

 public IList<T> GetAllByExpression(Expression<Func<T,bool>> expression)
        {
            using (ISession session = NHibernateHelper.OpenSession())
            {
                return session.Query<T>().Where(expression).ToList();
            }
        }

at some later point in the code I try to access location.attachments collection but get an exception:

{"Initializing[Location#543c7367-08fa-4469-a252-8e2c6163be22]-failed

to lazily initialize a collection of role: Location.Attachments, no
session or session was closed”}

I understand the session is closed, but there is no much I can do.

I understand the session is closed, but there is no much I can do.

I have a dilema:
1) The code uses the collection after fetching from the DB and needs the session to still be open.
leaving the session open for the whole time is a bad practice. (without using).

2) Even if i leave the session open during the whole application runtime, I have build an n-tier web application.
The DAL should be general and not expose some implementation’s session (NH in this case).

3) Eager loading seems to be the solution, but it’s wastefull – as I’ll have to eager load all the data in case of an object with list of other objects in it. (And that can even continue recurrsively)

What should I do?

TIA

Answer 1

As Jason suggests, it’s because you’ve closed the session.

If you’re using Web Forms then you may want to look at Session Per Request, where a session is opened on the start of an Http Request, and closed on the end of the Http Request. (global asax for those RequestBegin/RequestEnd events)

That will enable you to lazy-load.

Alternatively you can Eagerly fetch the data you want by implementing a Fetching Strategy that you pass in to the repository to say “grab this relating data while getting me the root”.