public ActionResult EnrollUser(EnrollmentModel model)
        {
            if ((model.StartDate != null) && (model.EndDate != null) &&
                (model.StartDate.Value.CompareTo(model.EndDate) > 0))
            {
                ModelState.AddModelError("EndDate", "Start date must preceed end date.");
                return View("Index", model);
            }

            var course = this.Data.Courses.GetById(model.CourseId);
            var user = this.Data.Users.All().FirstOrDefault(u => u.Id == model.Id);
            var courseInstance = new CourseInstance()
            {
                User = user,
                Course = course,
                Mark = null,
                StartDate = model.StartDate,
                EndDate = model.EndDate
            };
            this.Data.CourseInstances.Add(courseInstance);
            this.Data.SaveChanges();
            model.Message = "User enrolled";

            return View("Index", model);
        }
        public ActionResult Enroll(int id)
        {
            var course = this.Data.Courses.GetById(id);

            var user = this.Data.Users.All().FirstOrDefault(u=>u.UserName == User.Identity.Name);
            var courseInstance = new CourseInstance()
            {
                User = user,
                Course = course,
                Mark = null,
                StartDate = DateTime.Now,
                EndDate = DateTime.Now.AddDays(10)
            };

            this.Data.CourseInstances.Add(courseInstance);
            this.Data.SaveChanges();

            return RedirectToAction("Index");
        }