Why can't I insert record with foreign key in a single server request?

Apperantly using UNSIGNED BIGINT causes this problem. When I switched to SIGNED BIGINT everything worked as it supposed to.

I tried to do this "the right way":

And then I wrote this little test app to scan a directory, store the directory and all its files in two tables:

static void Main(string[] args)
{
   string directoryName = args[0];

   if(!Directory.Exists(directoryName))
   {
      Console.WriteLine("ERROR: Directory '{0}' does not exist!", directoryName);
      return;
   }

   using (testEntities entities = new testEntities())
   {
      StoredDir dir = new StoredDir{ DirName = directoryName };
      entities.AddToStoredDirSet(dir);

      foreach (string filename in Directory.GetFiles(directoryName))
      {
         StoredFile stFile = new StoredFile { FileName = Path.GetFileName(filename), Directory = dir };
         entities.AddToStoredFileSet(stFile);
      }

      try
      {
         entities.SaveChanges();
      }
      catch(Exception exc)
      {
         string message = exc.GetType().FullName + ": " + exc.Message;
      }
   }
}

As you can see, I only have a single call to .SaveChanges() at the very end - this works like a charm, everything's as expected.

Something about your approach must be screwing up the EF system.....