public ActionResult New()
        {
            var viewModel = new ActivityCategoriesViewModel()
            {
                Categories = _categoryRepository.GetAll()
            };

            return(View("ActivityForm", viewModel));
        }
        public ActionResult Edit(int?id)
        {
            var activity = _activityRepository.GetById(id);

            if (activity == null)
            {
                return(HttpNotFound());
            }

            var viewModel = new ActivityCategoriesViewModel(activity)
            {
                Categories = _categoryRepository.GetAll()
            };

            return(View("ActivityForm", viewModel));
        }