/// <summary>
 /// Validates a comic book on the server
 /// before adding a new record or updating an existing record.
 /// </summary>
 /// <param name="comicBook">The comic book to validate.</param>
 private void ValidateComicBook(ComicBook comicBook)
 {
     // If there aren't any "SeriesId" and "IssueNumber" field validation errors...
     if (ModelState.IsValidField("ComicBook.SeriesId") &&
         ModelState.IsValidField("ComicBook.IssueNumber"))
     {
         // Then make sure that the provided issue number is unique for the provided series.
         if (_comicBooksRepository.SeriesHasIssueNumber(comicBook.Id, comicBook.SeriesId, comicBook.IssueNumber))
         {
             ModelState.AddModelError("ComicBook.IssueNumber",
                                      "The provided Issue Number has already been entered for the selected Series.");
         }
     }
 }