/// <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.HasArtistRoleCombination(viewModel.ComicBookId, viewModel.ArtistId, viewModel.RoleId))
{
ModelState.AddModelError("ArtistId",
"This artist and role combination already exists for this comic book.");
}
}
}