private static Author GetOrCreateAuthor(string authorName, BookStoreEntities context)
        {
            if (string.IsNullOrEmpty(authorName))
            {
                return null;
            }

            var authorInDB = context.Authors.Where(x => x.FullName.ToLower() == authorName.ToLower()).FirstOrDefault();

            if (authorInDB != null)
            {
                return authorInDB;
            }

            var newAuthor = new Author();
            newAuthor.FullName = authorName;
            
            return newAuthor;
        }
        private static void AddBook(List<string> authorNames, string title, string isbn, decimal price, string website)
        {
            BookStoreEntities context = new BookStoreEntities();
            Book newBook = new Book()
            {
                Title = title,
                ISBN = isbn,
                Price = price,
                Website = website
            };


            foreach (var authorName in authorNames)
            {
                var authorInDB = GetOrCreateAuthor(authorName, context);
                newBook.Authors.Add(authorInDB);
            }

            context.Books.Add(newBook);
            if (newBook.Authors.Count == 0)
            {
                throw new ArgumentException("Author is required");
            }

            context.SaveChanges();
        }