public async Task <IActionResult> Get(string shortUrl)
        {
            var id = Shortener.Decode(shortUrl);

            if (id == -1)
            {
                return(NotFound());
            }
            var mapping = await dbContext.Urls.FindAsync(id);

            if (mapping == null)
            {
                return(NotFound());
            }

            return(new RedirectResult(mapping.Url));
        }