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[HttpPost]
public ActionResult Create(StudentViewModel studentViewModel)
{
    if (!ModelState.IsValid) // added ! so that you can do something if it is valid other than redisplay the Create View with the model 
    {
        return View(studentViewModel);
    }

        Student s = new Student()
         {
             Name =studentViewModel.Student.Name,
             Speciality = studentViewModel.Student.Speciality,
             DateOfJoinig = studentViewModel.Student.DateOfJoinig,
             Qualification = studentViewModel.Student.Qualification,
             Email = studentViewModel.Student.Email

         };

         db.Students.Add(s);
         db.SaveChanges(); // Post the changes with the student and save changes so you have the correct studentId PrimaryKey value from the database. Note: EF should update your object with the correct Id on SaveChanges() 

        StudentAddress sa = new StudentAddress()
        {
            StudentId= studentViewModel.Student.StudentId,
            Address = studentViewModel.StudentAddress.Address,
            Area = studentViewModell.StudentAddress.Area,
            City = studentViewModel.StudentAddress.City,
            State = studentViewModel.StudentAddress.State,
            Pincode = studentViewModel.StudentAddress.Pincode,
            Phone = studentViewModel.StudentAddress.Phone,
            Mobile = studentViewModel.StudentAddress.Mobile

        };

        StudentPhoto sp = new StudentPhoto()
        {
            StudentId= studentViewModel.Student.StudentId,
            Photo = studentViewModel.StudentPhoto.Photo

        };    
        // don't add it again!  db.Students.Add(s);
        db.StudentAddress.Add(sa);
        db.StudentPhoto.Add(sp);

        db.SaveChanges();
        return RedirectToAction("Home");
}