Hello,

I am just starting to use NHibernate, but I have a design question.

Take a standard web application where I would need to update an object

Http Request 1: get object you need to update
Http Request 2: update object, for this I see 2 ways:

1) Do a Load() of the object, update the values, save (but if I get it correctly then you'd do an unnecessary select on the database before updating)

2) Use the Update (or SaveOrUpdate method), no select on database, but if your object is by any chance in your session already it will throw an exception (as specified by the doc). I read in the documention and examples that it's usual to use a single session for an entire http request so there is a chance the object will already be in session.

I'm pretty sure number 2 is the way to go since every example I've looked at uses this almost, but how can I prevent the error I get ? In a lot of examples they don't seem to be ckecking for it and just assume it won't be in session already. What would be the best way to design this ?

Some extra info: I use a singleton for my sessionfactory to pass out sessions (as in the quickstart example). (going to change that to a HttpModule though, I got a nice example in the Benday Bugtracker example).

Thanks in advance,

Jorn

With scenario #2, it is theoretically true that you could be updating an entity that is already in the session and cause an exception. In practice, especially if you are using an HttpModule to open and close a session with every request (a.k.a. Session-In-View pattern), you are unlikely to have that problem since you get a new session with every request. It may not be comforting enough to know that it is only unlikely to give you problem. In that case, you can use the SaveOrUpdateCopy() method.

Thank you for your reply, that will indeed fix my problem to not get an exception anymore.

But won't this in most cases also result in a select of the object to the database then, just like the Load() in method number 1) I stated ?

Thinking about it, won't this always be the case with NHibernate in a web application ? For every update you need to do in a web application, NHibernate will always have to select the object before updating it ? Is there a way to avoid this ?

I don't believe so. NHibernate only compares the type and id of the object. By virtue of calling SaveOrUpdateCopy(), you are in essence telling NHibernate to overwrite the entity object in the session with the entity object you just gave it.



Not if you keep a reference of the entity around (e.g. in the ASP.NET session) from request to request. In this case, you would be working with detached entities in the postbacks. Have a read of the "Updating Objects" section of the docs:
http://www.hibernate.org/hib_docs/nhibe ... a-updating

Ok thanks :)

I had read that part of the doc, and I guess I was confused by it stating



I then assumed it woud load it from the database (since sometimes my object will already exist, sometimes not)