ASP.Net -添加一个包含其他对象的新对象，但被包含的对象变为空

我试图添加一个课程对象到我的数据库，在我的形式，我有3个其他对象，可以包含在课程对象。在我的表单中，其他3个对象显示，但当它返回到控制器时，3个对象值为null。

如果需要更多的信息，请告诉我。

为什么Teacher, Student, GradedWork值返回为空?

我的控制器创建方法

public ActionResult Create()
    {
        var addForm = new CourseAddForm();
        addForm.Teachers = m.GetAllTeachersAsList();
        addForm.Students = m.GetAllStudentsAsList();
        addForm.GradedWorks = m.GetAllGradedWorkAsList();
        return View(addForm);
    }
    //
    // POST: /Course/Create
    [HttpPost]
    public ActionResult Create(CourseAdd newItem)
    {
        if (ModelState.IsValid)
        {
            var addedItem = m.AddCourse(newItem);
            if (addedItem == null)
            {
                return RedirectToAction("Index");
            }
            else
            {
                return RedirectToAction("Details", new { id = addedItem.Id });
            }

        }
        else
        {
            var addForm = Mapper.Map<CourseAddForm>(newItem);
            addForm.Teachers = m.GetAllTeachersAsList();
            addForm.Students = m.GetAllStudentsAsList();
            addForm.GradedWorks = m.GetAllGradedWorkAsList();
            return View(addForm);
        }
    }

My view models

public class CourseAddForm : CourseAdd
{
    [Display(Name = "Teacher")]
    public ICollection<TeacherList> Teachers { get; set; }
    [Display(Name = "Student")]
    public ICollection<StudentList> Students { get; set; }
    [Display(Name = "Graded Works")]
    public ICollection<GradedWorkList> GradedWorks { get; set; }
    public CourseAddForm()
    {
        this.Teachers = new List<TeacherList>();
        this.Students = new List<StudentList>();
        this.GradedWorks = new List<GradedWorkList>();
    }

}
public class CourseAdd
{
    //[Key]
    [Display(Name = "Course Code")]
    public int CourseCode { get; set; }
    // [Required]
    [Display(Name = "Name")]
    public string Name { get; set; }
    //  [Required]
    [Display(Name = "Semester")]
    public string Semester { get; set; }
    // [Required]
    [Display(Name = "Section Id")]
    public string SectionId { get; set; }
    [Display(Name = "Teacher")]
    public ICollection<TeacherList> Teachers { get; set; }
    [Display(Name = "Student")]
    public ICollection<StudentList> Students { get; set; }
    [Display(Name = "Graded Works")]
    public ICollection<GradedWorkList> GradedWorks { get; set; }
}

public class CourseBase : CourseAdd
{
    [Key]
    public int Id { get; set; }
}

public class CourseBaseAddOn
{
    [Key]
    public int Id { get; set; }
    //[Key]
    [Display(Name = "Course Code")]
    public int CourseCode { get; set; }
    // [Required]
    [Display(Name = "Name")]
    public string Name { get; set; }
    //  [Required]
    [Display(Name = "Semester")]
    public string Semester { get; set; }
    // [Required]
    [Display(Name = "Section Id")]
    public string SectionId { get; set; }
    [Display(Name = "Teacher")]
    public ICollection<TeacherList> Teachers { get; set; }
    [Display(Name = "Student")]
    public ICollection<StudentList> Students { get; set; }
    [Display(Name = "Graded Works")]
    public ICollection<GradedWorkList> GradedWorks { get; set; }
}

My manager method

public CourseBaseAddOn AddCourse(CourseAdd newItem)
    {
        Course course = Mapper.Map<Course>(newItem);
        ds.Courses.Add(course);
        ds.SaveChanges();
        return Mapper.Map<CourseBaseAddOn>(course);
    }

我的形式

! (https://i.stack.imgur.com/c7abx.jpg)