NUnit NHibernate and Moq unit test throwing NHibernate.MappingException

I'm using Ayende's code from his blog on unit testing with NHibernate and it calls for testing with SQLite, code as follows:

using System;
using System.Reflection;
using NHibernate;
using NHibernate.Cfg;
using NHibernate.Dialect;
using NHibernate.Driver;
using NHibernate.Tool.hbm2ddl;
using Environment = NHibernate.Cfg.Environment;

namespace WebAPI.Tests
{
    /// <summary>
    /// http://ift.tt/1q8TEN7
    /// </summary>
    public class InMemoryDatabaseTest : IDisposable
    {
        private static Configuration _configuration;
        private static ISessionFactory _sessionFactory;
        protected ISession Session;

        public InMemoryDatabaseTest(Assembly assemblyContainingMapping)
        {
            if (_configuration == null)
            {
                _configuration = new Configuration()
                    .SetProperty(Environment.ReleaseConnections, "on_close")
                    .SetProperty(Environment.Dialect, typeof(SQLiteDialect).AssemblyQualifiedName)
                    .SetProperty(Environment.ConnectionDriver, typeof(SQLite20Driver).AssemblyQualifiedName)
                    .SetProperty(Environment.ConnectionString, "data source=:memory:")
                    .AddAssembly(assemblyContainingMapping);

                _sessionFactory = _configuration.BuildSessionFactory();
            }

            Session = _sessionFactory.OpenSession();

            new SchemaExport(_configuration).Execute(false, true, false);
        }

        public void Dispose()
        {
            Session.Dispose();
        }
    }
}

My test class is:

[TestFixture]
public class FilmsControllerTests : InMemoryDatabaseTest
{
    public FilmsControllerTests() : base(typeof(Film).Assembly)
    {
    }

    static readonly Guid ValidFilmId = new Guid("35d3ee0f5ee011bd8822ed629e5");

    [Test]
    public void CanSaveAndLoadFilm()
    {
        object id;

        using (var tx = Session.BeginTransaction())
        {
            id = Session.Save(new PdfFilm
            {
                IsEnabled = true,
                Id = ValidFilmId,
                PremiumLevel = 1,
                PublishDate = new DateTime(20121010),
                Pdf = new Document(new byte[byte.Parse(" 162")], new DateTime(20121010)),
                Title = "jel",
                Authors = "smel",

            });

            tx.Commit();
        }

        Session.Clear();


        using (var tx = Session.BeginTransaction())
        {
            var Film = Session.Get<PdfFilm>(id);

            Assert.AreEqual(new DateTime(2012, 10, 10), Film.PublishDate);

            tx.Commit();
        }
    }

And Model:

public abstract class Film : Entity<Film>

Which has a nested class:

public class PdfFilm: Film

And starts it's mapping config in the same file as the parent as:

<subclass name="PdfFilm" discriminator-value="Pdf">

The NUnit unit test returns:

NHibernate.MappingException : No persister for: Core.PdfFilm

I've checked the properties of the Film mapping file and it's set as embedded resource - could anyone please also confirm if this is the simplest way of testing and also any idea what the error message means?

Stack trace:

at NHibernate.Impl.SessionFactoryImpl.GetEntityPersister(String entityName)
at NHibernate.Impl.SessionImpl.GetEntityPersister(String entityName, Object obj)
at NHibernate.Event.Default.AbstractSaveEventListener.SaveWithGeneratedId(Object entity, String entityName, Object anything, IEventSource source, Boolean requiresImmediateIdAccess)
at NHibernate.Event.Default.DefaultSaveOrUpdateEventListener.SaveWithGeneratedOrRequestedId(SaveOrUpdateEvent event)
at NHibernate.Event.Default.DefaultSaveOrUpdateEventListener.EntityIsTransient(SaveOrUpdateEvent event)
at NHibernate.Event.Default.DefaultSaveOrUpdateEventListener.OnSaveOrUpdate(SaveOrUpdateEvent event)
at NHibernate.Impl.SessionImpl.FireSave(SaveOrUpdateEvent event)
at NHibernate.Impl.SessionImpl.Save(Object obj)

via Chebli Mohamed