public ActionResult Create()
        {
            var categories = _categoryService.GetAllCategories().ToList();

            var categorySelectList = categories.Select(c => new SelectListItem
                {
                    Text = c.Name,
                    Value = c.Id.ToString()
                });

            var viewModel = new TransactionViewModel ();
            viewModel.CategorySelectList = categorySelectList.ToList ();

            return View ("Create", viewModel);
        }
        public ActionResult Edit(int id)
        {
            var transaction = _transactionService.GetTransactionById (id: id);
            var categories = _categoryService.GetAllCategories().ToList();

            var categorySelectList = categories.Select(c => new SelectListItem
                {
                    Text = c.Name,
                    Value = c.Id.ToString(),
                    Selected = (c.Id == transaction.Category_Id)? true: false,
                });

            var viewModel = new TransactionViewModel ()
            {
                Transaction = transaction,
                CategorySelectList = categorySelectList.ToList()
            };
            return View ("Edit", viewModel);
        }