public async Task <bool> AddBookToSubscription(int subscriptionId, int bookId) { var book = _bookRepository.GetBook(bookId); var subscription = _subscriptionRepository.GetSubscriptionBySubscriptionId(subscriptionId); // if book already exists in subscription AND subscription does not exist if (book != null && subscription == null) { return(false); } return(await _bookRepository.AddBookToSubscription(subscriptionId, bookId)); }