public Task <DiscountDTO> UpdateAsync(DiscountDTO discountDTO)
{
var existingDiscount = Discounts.Get(discountDTO.Id);
if (existingDiscount == null)
{
throw new Exception(string.Format(ERROR_DISCOUNT_NOT_FOUND, discountDTO.Id));
}
CheckValidProductSKU(discountDTO);
Discounts.AddOrUpdate(discountDTO.ToModel());
return(Task.FromResult <DiscountDTO>(discountDTO));
}