entity update

[HttpPost]
public ActionResult Edit(Models.MathClass mathClassModel)
{
    //get current entry from db (db is context)
    var item = db.Entry<Models.MathClass>(mathClassModel);
 
    //change item state to modified
    item.State = System.Data.Entity.EntityState.Modified;
 
    //load existing items for ManyToMany collection
    item.Collection(i => i.Students).Load();
 
    //clear Student items          
    mathClassModel.Students.Clear();
 
    //add Toner items
    foreach (var studentId in mathClassModel.SelectedStudents)
    {
        var student = db.Student.Find(int.Parse(studentId));
        mathClassModel.Students.Add(student);
    }                
 
    if (ModelState.IsValid)
    {
       db.SaveChanges();
       return RedirectToAction("Index");
    }
 
    return View(mathClassModel);
}