Category: Coding

Do you see any (or all) of these exceptions (NHibernate.ADOException) in you NHibernate-based web application?

could not execute query
failed to lazily initialize a collection, no session or session was closed
Transaction not successfully started
The server failed to resume the transaction. Desc:86000002dc.

If you do, it might be a good idea to make sure you don’t store any ISession-references anywhere. If you do there’s a great risk that something goes wrong if two threads (i.e. two requests) are using the session at the same time as the NHibernate session implementation is not thread safe.

Instead, do this when an ISession is needed:

ISession session = SessionFactory.Factory.GetCurrentSession();

That code will get the session from the current context (which of course must be updated by your code, look here for more info).

/Emil

If you’re using ICriteria to write queries in NHibernate and you’re having citeria on items in a one-to-many relation (e.g. retrieve all departments with at least one employee named “John”) then you’re likely to have experienced that too many objects are returned (at least more than expected). In the example, all departments with multiple “Johns” will be returned once per matching employee. This is probably not what’s expected, so here’s how to fix it:

// The 'criteria' variable contains criteria on Department

// Create alias to use for employee conditions
ICriteria aliasCriteria = criteria.CreateAlias("Employees", "pr");

// Add criteria on the alias...

// We only want distinct root objects returned
criteria.SetResultTransformer(Transformers.DistinctRootEntity);

The trick is to use Transformers.DistinctRootEntity (at least in NHibernate 2.1). In earlier versions you may want to try something like this:

criteria.SetResultTransformer(new DistinctRootEntityResultTransformer());

Related to this is also how to write criteria that counts the results without actually retrieving them, which can be very useful for paging the result set (i.e. only read a subset of the results at a time). Normally you would write something like this:

criteria.SetProjection(Projections.Count("Id"));
int numberOfItems = (int)criteria.UniqueResult();

However, this suffers from the same problem as described above. To get the correct result, do this:

criteria.SetProjection(Projections.CountDistinct("Id"));
int numberOfItems = (int)criteria.UniqueResult();

Problem solved!

Credits to my colleague Jon for helping out with these rather obscure problem solutions!

/Emil

In later versions of NHibernate it’s possible to separate NHibernate configuration from normal application configuration (which is normally done in App.config/Web.config) by using a separate file called hibernate.cfg.xml.

Works great, but there is one thing to keep in mind in ASP.Net applications, namely that it should be placed in the bin/ folder, not in the root folder fo the application! Took me quite some time to figure out! So if you have the same problem as me, just move it to that folder 🙂

/Emil