Intermittent error Linq to NHibernate Sequence Contains no Elements

Question

This may sound like a duplicate question, but I don't believe it is.

This is the error that I'm getting, which sounds very common. But I'm only getting this error intermittently. For the number of users that I get to my site, I'm estimating that this only occurs about 5% of the time.

Here's the code:

private TopWrestlers FillTopWrestlers()
{
    try
    {
        var mostRecent = _wrestlerRankingRepo.Query().OrderByDescending(wr => wr.Season).First().Season;
        var currentSeason = _configService.GetCurrentSeason();

        if (mostRecent > currentSeason)
            return null;

        var tws =
            _wrestlerRankingRepo.GetRankedWrestlers(currentSeason)
                .Where(wr => wr.Rank <= 3)
                .OrderBy(wr => wr.Rank)
                .Select(wr => wr.Wrestler);

        return new TopWrestlers
        {
            _125 = tws.FirstOrDefault(w => w.Roster.WeightClass == 125),
            _133 = tws.FirstOrDefault(w => w.Roster.WeightClass == 133),
            _141 = tws.FirstOrDefault(w => w.Roster.WeightClass == 141),
            _149 = tws.FirstOrDefault(w => w.Roster.WeightClass == 149),
            _157 = tws.FirstOrDefault(w => w.Roster.WeightClass == 157),
            _165 = tws.FirstOrDefault(w => w.Roster.WeightClass == 165),
            _174 = tws.FirstOrDefault(w => w.Roster.WeightClass == 174),
            _184 = tws.FirstOrDefault(w => w.Roster.WeightClass == 184),
            _197 = tws.FirstOrDefault(w => w.Roster.WeightClass == 197),
            _285 = tws.FirstOrDefault(w => w.Roster.WeightClass == 285)
        };
    }
    catch (Exception ex)
    {
        _errorService.LogError(new Exception("Exception occurred trying to retrieve the top wrestlers for each weight on the home page. " + ex.Message, ex));
        return null;
    }
}

I'm getting the error on the very first line:

_wrestlerRankingRepo.Query().OrderByDescending(wr => wr.Season).First().Season;

I KNOW that repo has data. Especially since it returns just fine 95% of the time.

Any help on how I can figure out how to fix this bug? I'm not even able to re-create the issue. This is on the homepage of the site. So I don't believe it has to do with a new session firing up....I'm at a loss here.

Here's the base repo code:

public abstract class BaseRepository<TRecord, TMap> : IBaseRepository<TRecord>
    where TRecord : class, IEntity
    where TMap : ClassMap<TRecord>, new()
{
    private static Member _primaryKeyMember;
    protected ISession Session;
    protected IUserIdentity UserIdentity;

    public BaseRepository(ISession session)
    {
        Session = session;
    }

    public BaseRepository(ISession session, IUserIdentity userIdentity)
    {
        Session = session;
        UserIdentity = userIdentity;
    }

    public void Delete(TRecord obj)
    {
        Session.Delete(obj);
        Session.Flush();
    }

    public void Save(TRecord value)
    {
        Session.SaveOrUpdate(value);
        Session.Flush();
    }

    public void Save(IEnumerable<TRecord> values)
    {
        using (ITransaction sessionTransaction = Session.BeginTransaction())
        {
            try
            {
                foreach (TRecord value in values)
                    Session.SaveOrUpdate(value);

                sessionTransaction.Commit();
                Session.Flush();
            }
            catch
            {
                sessionTransaction.Rollback();
                throw;
            }
        }
    }

    public virtual IQueryable<TRecord> Query()
    {
        return Session.Query<TRecord>();
    }
}

public interface IBaseRepository<TRecord>
    where TRecord : IEntity
{
    void Delete(TRecord obj);

    void Save(TRecord value);
    void Save(IEnumerable<TRecord> values);
    IQueryable<TRecord> Query();
}

And here's the WrestlerRankingRepo code:

public class WrestlerRankingRepository : BaseRepository<WrestlerRanking, WrestlerRankingMap>, IWrestlerRankingRepository
{
    public WrestlerRankingRepository(ISession session) : base(session)
    {
    }

    public IQueryable<WrestlerRanking> GetRankedWrestlers(int season)
    {
        return base.Query()
            .Where(wr => wr.Rank != null)
            .Where(wr => wr.Season == season)
            .Where(wr => wr.IsCurrent)
            .Where(wr => !wr.Wrestler.LastName.StartsWith("("));
    }

    public IQueryable<WrestlerRanking> GetRankedWrestlers(int season, int weight)
    {
        return GetRankedWrestlers(season)
            .Where(wr => wr.WeightClass == weight);
    }

    public IQueryable<WrestlerRanking> GetRankedWrestlersWithMatches(int season)
    {
        return GetRankedWrestlers(season);
        // for some reason it hates this: .Where(w => w.CurrentStats != null)
    }

    public IQueryable<WrestlerRanking> GetRankedWrestlersWithMatches(int season, int weight)
    {
        return GetRankedWrestlers(season)
            .Where(w => w.WeightClass == weight);
        // for some reason it hates this: .Where(w => w.CurrentStats != null)
    }
}

public interface IWrestlerRankingRepository : IBaseRepository<WrestlerRanking>
{
    IQueryable<WrestlerRanking> GetRankedWrestlers(int season);
    IQueryable<WrestlerRanking> GetRankedWrestlers(int season, int weight);
    IQueryable<WrestlerRanking> GetRankedWrestlersWithMatches(int season);
    IQueryable<WrestlerRanking> GetRankedWrestlersWithMatches(int season, int weight);
}

SessionFactory code:

public static class SessionFactory
{
    private static ISessionProvider _instance;

    public static ISessionProvider Instance
    {
        get
        {
            if (_instance == null)
            {
                string sessionClass = ConfigurationManager.AppSettings["SessionProvider"];

                if (string.IsNullOrWhiteSpace(sessionClass))
                    throw new ConfigurationErrorsException("Session Provider must be specified in the app.config");

                _instance = (ISessionProvider)Activator.CreateInstance(Type.GetType(sessionClass));
            }

            return _instance;
        }
    }

    [Obsolete]
    public static ISession GetCurrentSession()
    {
        return GetSession();
    }

    public static ISession GetSession()
    {
        return Instance.GetSession();
    }

    public static string GetConnectionString()
    {
        return ConfigurationManager.ConnectionStrings["WrestleStat"].ConnectionString;
    }

    public static IPersistenceConfigurer BuildConfig()
    {
        return MsSqlConfiguration.MsSql2008.ConnectionString(GetConnectionString());
    }

    public static void BuildMappings(MappingConfiguration mappings)
    {
        mappings.FluentMappings.AddFromAssembly(Assembly.GetExecutingAssembly());
    }
}

public class SqlStatementInterceptor : EmptyInterceptor
{
    public override SqlString OnPrepareStatement(SqlString sql)
    {
        Trace.WriteLine(sql.ToString());
        return sql;
    }
}

public interface ISessionProvider
{
    ISession GetSession();
    ISession OpenSession();
    void CloseSession();
}

Here's the Ninject impelementation:

private static void RegisterServices(IKernel kernel)
    {
        kernel.Bind(scanner => scanner
                                   .From(new List<Assembly>
                                             {
                                                 typeof (HomeController).Assembly,
                                                 typeof (Match).Assembly
                                             })
                                   .SelectAllClasses()
                                   .BindAllInterfaces()
                                   .Configure(b => b.InTransientScope())
            );

        kernel.Rebind<ISession>().ToMethod(icontext => SessionFactory.GetSession()).InRequestScope();
        //kernel.Rebind<IUserIdentity>().ToMethod(i => MvcApplication.GetWebIdentity()).InRequestScope();
    }

Another spot that's relevant...

public class CoreWebSessionModule : IHttpModule
{
    public void Init(HttpApplication context)
    {
        context.BeginRequest += OpenSession;
        context.EndRequest += CloseSession;
    }

    private static void OpenSession(object sender, EventArgs e)
    {
        SessionFactory.Instance.OpenSession();
    }

    private static void CloseSession(object sender, EventArgs e)
    {
        SessionFactory.Instance.CloseSession();
    }

    public void Dispose()
    {
    }
}

One more

public class CoreWebSessionProvider : ISessionProvider
    {
        private static ISessionFactory _holder;

        private static ISessionFactory MySessionFactory
        {
            get
            {
                if (_holder == null)
                {
                    _holder = GetFluentConfiguration().BuildSessionFactory();
                }

                return _holder;
            }
        }

        public ISession GetSession()
        {
            return HttpContext.Current != null ? MySessionFactory.GetCurrentSession() : null;
        }

        public ISession OpenSession()
        {
            var session = MySessionFactory.OpenSession();
            ManagedWebSessionContext.Bind(HttpContext.Current, session);
            return session;
        }

        public void CloseSession()
        {
            var session = ManagedWebSessionContext.Unbind(HttpContext.Current, MySessionFactory);

            if (session != null)
            {
                if (session.Transaction != null && session.Transaction.IsActive)
                    session.Transaction.Rollback();
                //else
                //    session.Flush();

                if (session.IsOpen)
                    session.Close();
            }
        }

        private static FluentConfiguration GetFluentConfiguration()
        {
            return
                Fluently.Configure()
                        .Database(SessionFactory.BuildConfig())
                        .ExposeConfiguration(BuildSchema)
                        .Mappings(SessionFactory.BuildMappings);
        }

        public static Configuration GetConfiguration()
        {
            return GetFluentConfiguration().BuildConfiguration();
        }

        private static void BuildSchema(Configuration config)
        {
            config.SetProperty("current_session_context_class", "managed_web");
        }

        public IStatelessSession GetStatelessSession()
        {
            return MySessionFactory.OpenStatelessSession();
        }
    }

Answer 1

From the NHibernate documentation (emphasis mine):

An ISessionFactory is an expensive-to-create, threadsafe object intended to be shared by all application threads. An ISession is an inexpensive, non-threadsafe object that should be used once, for a single business process, and then discarded

So you should not be sharing your session object around like this. If possible, create one only when needed and destroy it as soon as possible.

Some excellent further reading here that talks about context and lifetime, including NHibernate.

Answer 2

The problem is most likely using a single session,

public static ISession GetSession()
{
    return Instance.GetSession();
}

and since your web application uses multiple threads, and each time this will return the same session. By definition session is not thread safe.

Best option is to configure your DI to inject new session per request (if you are using DI), for example for Autofac

builder.Register(x => x.Resolve<ISessionFactory>().OpenSession())
    .InstancePerHttpRequest();