// GET: Courses/Edit/5
        public async Task <IActionResult> Edit(int?id)
        //tuka
        {
            if (id == null)
            {
                return(NotFound());
            }
            var course = _context.Course.Where(m => m.Id == id).Include(m => m.Students).First();


            course = await _context.Course.FindAsync(id);

            if (course == null)
            {
                return(NotFound());
            }
            //tuka
            CourseStudentsEditViewModel viewmodel = new CourseStudentsEditViewModel
            {
                Course           = course,
                StudentList      = new MultiSelectList(_context.Student.OrderBy(s => s.StudentId), "Id", "FullName"),
                SelectedStudents = course.Students.Select(sa => sa.StudentId)
            };


            ViewData["FirstTeacherId"]  = new SelectList(_context.Set <Teacher>(), "Id", "FullName", course.FirstTeacherId);
            ViewData["SecondTeacherId"] = new SelectList(_context.Set <Teacher>(), "Id", "FullName", course.SecondTeacherId);
            return(View(viewmodel)); //tuka
        }
        public async Task <IActionResult> Edit(int id, CourseStudentsEditViewModel viewmodel)  /*Edit(int id, [Bind("Id,Title,Credits,Semester,Programme,EducationLevel,FirstTeacherId,SecondTeacherId")] Course course)*/
        {
            if (id != viewmodel.Course.Id)
            {
                return(NotFound());
            }

            if (ModelState.IsValid)
            {
                try
                {
                    _context.Update(viewmodel.Course);
                    await _context.SaveChangesAsync();

                    IEnumerable <int>       listStudents = viewmodel.SelectedStudents;
                    IQueryable <Enrollment> toBeRemoved  = _context.Enrollment.Where(s => !listStudents.Contains(s.StudentId) && s.CourseId == id);
                    _context.Enrollment.RemoveRange(toBeRemoved);
                    IEnumerable <int> existStudents = _context.Enrollment.Where(s => listStudents.Contains(s.StudentId) && s.CourseId == id).Select(s => s.StudentId);
                    IEnumerable <int> newStudents   = listStudents.Where(s => !existStudents.Contains(s));
                    foreach (int studentId in newStudents)
                    {
                        _context.Enrollment.Add(new Enrollment {
                            StudentId = studentId, CourseId = id
                        });
                    }

                    //_context.Update(course);
                    await _context.SaveChangesAsync();
                }
                catch (DbUpdateConcurrencyException)
                {
                    if (!CourseExists(viewmodel.Course.Id))
                    {
                        return(NotFound());
                    }
                    else
                    {
                        throw;
                    }
                }
                return(RedirectToAction(nameof(Index)));
            }
            ViewData["FirstTeacherId"]  = new SelectList(_context.Set <Teacher>(), "Id", "FullName", viewmodel.Course.FirstTeacherId);
            ViewData["SecondTeacherId"] = new SelectList(_context.Set <Teacher>(), "Id", "FullName", viewmodel.Course.SecondTeacherId);
            return(View(viewmodel));
        }