public void FollowUser_UserIsAlreadyFollowing_ThrowAnInvalidOperationException()
        {
            _repository.Setup(repo => repo.Table).Returns(new List <Follow>
            {
                _follow
            }.AsQueryable());


            Assert.That(async() => await _followService.FollowUserAsync(_followeeId),
                        Throws.InvalidOperationException);

            _repository.Verify(repo => repo.AddAsync(It.IsAny <Follow>()), Times.Never);
        }