private void ValidateResource(Resource resource)
 {
     if (ModelState.IsValidField("Resource.URL"))
     {
         if (_resourcesRepository.LinkAlreadyExists(resource.Id, resource.URL, resource.SectionId))
         {
             ModelState.AddModelError("Resource.URL",
                                      "The provided URL has already been added for the selected section.");
         }
     }
 }