protected override IEnumerable<NeuralVectorFlow> DoGetNext(IndexSet indexes)
 {
     using (var ctx = new LeCunDataEntities())
     {
         var selectIndexes = indexes.ToDictionary(i => indexMap[i]);
         foreach (var sample in (from s in ctx.Samples
                                 where s.IsTraining == IsTraining && selectIndexes.Keys.Contains(s.ID)
                                 select s))
         {
             yield return ToFlow(selectIndexes[sample.ID], sample);
         }
     }
 }