public void RemoveRelationForAuthor(int bookId)
        {
            //var bookAuthors = _bookAuthorRepository.GetAll();
            //var relatedObjects = bookAuthors.Where(x => x.BookId == bookId);
            var bookAuthors    = _bookAuthorRepository.Query();
            var relatedObjects = bookAuthors.Where(x => x.BookId == bookId).ToList();

            foreach (var bookAuthor in relatedObjects)
            {
                _bookAuthorRepository.Delete(bookAuthor);
            }
        }
        public void Delete(int id)
        {
            _repository.Delete(id);
            //get related books etc.
            var relatedObjects = _bookAuthorRepository.GetAll().Where(x => x.AuthorId == id);

            //delete related books etc.
            foreach (var r in relatedObjects)
            {
                _bookAuthorRepository.Delete(r);
            }
        }