public ActionResult AddBook(int?id)
        {
            AddBookToStore model = new AddBookToStore();

            model.Books   = db.Books.ToList();
            model.StoreId = id ?? 0;
            return(View(model));
        }
        public ActionResult AddBook(AddBookToStore model)
        {
            var book  = db.Books.Find(model.BookId);
            var store = db.Stores.Find(model.StoreId);

            store.Books.Add(book);
            db.SaveChanges();
            return(View("Index", db.Stores.ToList()));
        }