@Override
 public Iterable<Article> getArticlesByCategory(Category category) {
   return repo.findByCategoryOrderByIdDesc(category);
 }
 @Override
 public void deleteArticle(long id) {
   repo.delete(id);
 }
 @Override
 public Article saveArticle(Article article) {
   return repo.save(article);
 }
 @Override
 public Article getArticleById(long id) {
   return repo.findOne(id);
 }
 @Override
 public Iterable<Article> listArticlesByTime() {
   return repo.findAllByOrderByIdDesc();
 }
 @Override
 public Iterable<Article> listAllArticles() {
   return repo.findAll();
 }