Cassandra: Argument types do not match

Question

I am a newbie to Cassandra and created a generic repository for my Cassandra database using linq. For my Single() method, I am passing the where criteria as a parameter.

This is my Single method:

Single(Expression<Func<T, bool>> exp)

This is the linq code I am using to query the cassandra database

public async Task<T> Single(Expression<Func<T, bool>> exp)
{
    return await GetTable.Where<T>(exp).FirstOrDefault().ExecuteAsync();
}

This is the method calling the single method

public override Task OnConnected()
{
    if (Context.User != null)
    {
        string userName = Context.User.Identity.Name;

        this.Groups.Add(userName, Context.ConnectionId);

        ***** this is the line with the issue ******
        Notification notification = Task.Run(() => _notificationRepository.Single(x => x.UserName.Equals(userName))).Result;

        if (notification == null)
        {
            _notificationRepository.CreateInstance(new NotificationUserMapping { Connections = new string[] { Context.ConnectionId }, UserName = Context.User.Identity.Name });
        }
        else if (!notification.Connections.Contains(Context.ConnectionId))
        {
            notification.Connections.Add(Context.ConnectionId);
            _notificationRepository.Save(notification);
        }
    }

    return base.OnConnected();
}

I keep getting a "System.AggregateException" of "Argument types do not match" and I am confused about where this could be coming from.

The database table columns:

id uuid PRIMARY KEY,
connections list<text>,
username text

and the c# poco:

[Table(ExplicitColumns = true)]
public class ConnectionMapping
{
    [Column("id")]
    public Guid Id { get; set; }
    [Column("connections")]
    public IList<string> Connections { get; set; }
    [Column("username")]
    public string UserName { get; set; }
}

and the exception:

   at System.Linq.Expressions.Expression.Condition(Expression test, Expression ifTrue, Expression ifFalse)
   at Cassandra.Mapping.MapperFactory.GetExpressionToGetColumnValueFromRow(ParameterExpression row, CqlColumn dbColumn, Type pocoDestType)
   at Cassandra.Mapping.MapperFactory.CreateMapperForPoco[T](RowSet rows, PocoData pocoData)
   at Cassandra.Mapping.MapperFactory.CreateMapper[T](RowSet rows)
   at Cassandra.Mapping.MapperFactory.<>c__DisplayClass1`1.<GetMapper>b__0(Tuple`2 _)
   at System.Collections.Concurrent.ConcurrentDictionary`2.GetOrAdd(TKey key, Func`2 valueFactory)
   at Cassandra.Mapping.MapperFactory.GetMapper[T](String cql, RowSet rows)
   at Cassandra.Mapping.Mapper.<>c__DisplayClass7`1.<FetchAsync>b__6(Statement s, RowSet rs)
   at Cassandra.Mapping.Mapper.<>c__DisplayClass2`1.<>c__DisplayClass4.<ExecuteAsyncAndAdapt>b__1(Task`1 t2)
   at Cassandra.Tasks.TaskHelper.DoNext[TIn,TOut](Task`1 task, Func`2 next)
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()
   at CompanyNamespace.CompanyDomain.NotificationRepository.<SingleByUserName>d__0.MoveNext()

What am I missing. I reveiwed the docs. and the mapping rules between Cassandra and c# and everything seems correct.

Answer 1

Through some experimentation, I found the answer. Come to find out Cassandra List-based collections do not map to c# lists, it map to c# IEnumerable objects instead.