public ActionResult New()
        {
            var membershipTypes = _context.MembershipTypes.ToList();
            var viewModel = new CustomerFormViewModel
            {
                MembershipTypes = membershipTypes
            };

            return View("CustomerForm", viewModel);
        }
        public ActionResult Edit(int id)
        {
            var customer = _context.Customers.SingleOrDefault(c => c.Id == id);
            if (customer == null)
            {
                return HttpNotFound();
            }
            var viewModel = new CustomerFormViewModel
            {
                Customer = customer,
                MembershipTypes = _context.MembershipTypes.ToList()
            };

            return View("CustomerForm", viewModel);
        }
        public ActionResult Save(Customer customer)
        {
            if (!ModelState.IsValid)
            {
                var viewModel = new CustomerFormViewModel
                {
                    Customer = customer,
                    MembershipTypes = _context.MembershipTypes.ToList()
                };
                return View("CustomerForm", viewModel);
            }

            if (customer.Id == 0)
            {
                // create a new customer
                customer.MembershipType = _context.MembershipTypes.Find(customer.MembershipType.Id);
                _context.Customers.Add(customer);

            }
            else
            {
                var customerInDb = _context.Customers.Single(c => c.Id == customer.Id);
                customerInDb.Name = customer.Name;
                customerInDb.Birthdate = customer.Birthdate;
                customerInDb.IsSubscribedToNewsletter = customer.IsSubscribedToNewsletter;
                customerInDb.MembershipType = _context.MembershipTypes.Find(customer.MembershipType.Id);

            }
            try
            {
                _context.SaveChanges();
            }
            catch (Exception ex)
            {

                return Content(ex.InnerException.ToString());
            }

            return RedirectToAction("Index", "Customers");
        }