public IActionResult AddGrade(GradeModel model) { if (ModelState.IsValid) { StudentModel student = StudentManager.GetByNameAndSurname(model.Name, model.Surname).ToModel(); if (student == null) { ModelState.AddModelError("pass", "In list of students don't exists student with this name and surname!"); } else { SubjectModel subject = SubjectManager.GetBySubjectName(model.SubjectName).ToModel(); if (subject == null) { ModelState.AddModelError("pass", "In list of subjects don't exists subject with this name!"); } else { var grade = GradeManager.GetByGradesInterval1To10().ToList(); if (grade != null) { ModelState.AddModelError("pass", "The grade aren't in interval 1 to 10!"); } else { GradeManager.AddNewGrade(model.Name, model.Surname, model.SubjectName, model.Grade, model.Description); return(RedirectToAction(nameof(GradeIndex))); } } } } return(View(model)); }