public async void get_user_by_email_should_return_user()
        {
            // Arrange
            const string email = "*****@*****.**";

            var userRepository = new Mock<IRepository<User>>();
            userRepository.Setup(x => x.FindOne(It.IsAny<Expression<Func<User, bool>>>())).Returns(new User { Email = email });

            // Act
            var userService = new UserService(userRepository.Object);
            var user = await userService.GetByEmail(email);

            // Assert
            Assert.NotNull(user);
            Assert.AreEqual(user.Email, email);
            userRepository.Verify(x => x.FindOne(It.IsAny<Expression<Func<User, bool>>>()), Times.Once);
        }