public ActionResult AddGrade(GradeAddGradeVM model)
        {
            if (model.GradeValue < 2 || model.GradeValue > 6)
            {
                ModelState.AddModelError("Error", "Grade must be 2 - 6");
                return View(model);
            }

            GradeRepository gRepo = new GradeRepository();
            Grade grade = new Grade();
            var test = gRepo.GetAll(filter: g => g.StudentID == model.StudentID && g.SubjectID == model.SubjectID).FirstOrDefault();
            if (test != null)
            {
                ModelState.AddModelError("Error", "Student already have grade");
                return View(model);
            }

            grade.StudentID = model.StudentID;
            grade.SubjectID = model.SubjectID;
            grade.GradeValue = model.GradeValue;

            gRepo.Save(grade);

            return RedirectToAction("StudentsGrades" + "/" + model.SubjectID, "Grade");
        }
        public JsonResult AddGradeToStudent(int gradevalue, int studentid, int subjectid)
        {
            GradeRepository gradeRepo = new GradeRepository();
            Grade grade = gradeRepo.GetAll(filter: g => g.StudentID == studentid && g.SubjectID == subjectid).FirstOrDefault();

            if (grade == null)
            {
                grade = new Grade();
                grade.StudentID = studentid;
                grade.GradeValue = gradevalue;
                grade.SubjectID = subjectid;

                gradeRepo.Save(grade);
                return Json(true);
            }
            return Json(false);
        }