public async Task BuyAsyncTestAsync()
        {
            try
            {
                var unitOfWork = new UnitOfWork(new LibraryDbContext());
                var bm         = new BookManager(unitOfWork);

                var book = (await unitOfWork.Books.GetAllAsync()).Take(1).Single();
                var user = (await unitOfWork.Users.GetAllAsync()).Take(1).Single();

                await bm.BuyAsync(book, user);

                Assert.IsTrue(true);
            }
            catch (Exception)
            {
                Assert.Fail();
            }
        }