/// <summary>
 /// Validates a comic book artist on the server before adding a new record.
 /// </summary>
 /// <param name="viewModel">The view model containing the values to validate.</param>
 private void ValidateComicBookArtist(ComicBookArtistsAddViewModel viewModel)
 {
     // If there aren't any "ArtistId" and "RoleId" field validation errors...
     if (ModelState.IsValidField("ArtistId") && ModelState.IsValidField("RoleId"))
     {
         // Then make sure that this artist and role combination doesn't already exist for this comic book.
         if (_comicBooksRepository.FindDuplicateArtistRoleCombination(viewModel.ComicBookId, viewModel.ArtistId, viewModel.RoleId))
         {
             ModelState.AddModelError("ArtistId",
                                      "This artist and role combination already exists for this comic book.");
         }
     }
 }