Check out a copy of the CodeCampServer source (svn).  CodeCampServer is easy to work with because it has good tests on an already-working NHibernate implementation.

Check out a copy of NH Contrib (svn), open the NHibernate.Linq solution in Visual Studio and build. You may have to remove the compiler flag that indicates .NET 3.5 sp1 is available.

Open the CodeCampServer.sln and remove the NHibernate.dll reference from the DataAccess and IntegrationTests projects.  Add a reference to NHibernate.dll from the NHibernate.Linq bin directory.  Build and run tests.  I had to play with the DatabaseTesterBase.recreateDatabase() method because I think are are still some bugs in Export():

public static void recreateDatabase()
{
    var exporter = new SchemaExport(new HybridSessionBuilder().GetConfiguration());
    exporter.Drop(false, true);
    exporter.Create(false, true);
}

Now add a reference to NHibernate.Linq.dll in the DataAccess project. You can start writing queries and check your work with the integration tests that already exist. Here are a few simple queries for ConferenceRepository:

public Conference[] GetAllConferences()
{
    ISession session = getSession();
    var query = from conference in session.Linq<Conference>() select conference;
    return query.ToList().ToArray();
}

public Conference GetConferenceByKey(string key)
{
    ISession session = getSession();
    var query = from conference in session.Linq<Conference>()
                where conference.Key == key
                select conference;
    return query.Distinct().Single();
}

public Conference GetFirstConferenceAfterDate(DateTime date)
{
    ISession session = getSession();
    var query = from conference in session.Linq<Conference>()
                where conference.StartDate >= date
                orderby conference.StartDate
                select conference;
    return query.First();
}

public Conference GetMostRecentConference(DateTime date)
{
    ISession session = getSession();
    var query = from conference in session.Linq<Conference>()
            where conference.EndDate <= date
            select conference;
    query.OrderByDescending(conference => conference.EndDate);
    return query.First();
}

Database trigger

• See Jon Galloway’s post on Auditing with SQL Server
• To generate an audit trigger in oracle you can do something similar.

NH Interceptor (per NH in action and the NH docs)

NH Listener (per the Hibernate docs)

Domain model

Because auditing is a simple application of interceptors there is a lot of guidance online about that approach, and using an interceptor was my first attempt at auditing.

But since the concept of an audit entry is important to the application (and the users!), it makes sense to promote this concept to a domain object.  Reporting is simple and the behavior is easier to manage.  Create a class called AuditEntry (with supporting mapping file) and hang collections of AuditEntry from the entity. Manage this collection in regular behavior:

public virtual void ChangeStatus(Employee employee, DateTime time, OrderStatus status)
{
    if(this.Status != status)
    {
        AuditEntry entry = new AuditEntry(employee, time, this.Status, status);
        _auditEntries.Add(entry);
    }
    Status = status;
}

After having gone down the database trigger path and the interceptor path and encountering friction I now recommend the domain model approach.