Asp.net 有人可以解释一下这个Eager 加载示例吗

我正在阅读Microsft关于ASP.NETMVC5和EF6的教程，链接是text. 现在我对下面的代码感到困惑.

public ActionResult Index(int? id, int? courseID)
{
    var viewModel = new InstructorIndexData();
    viewModel.Instructors = db.Instructors
        .Include(i => i.OfficeAssignment)
        .Include(i => i.Courses.Select(c => c.Department))   
        .OrderBy(i => i.LastName);

    if (id != null)
    {
        ViewBag.InstructorID = id.Value;
        viewModel.Courses = viewModel.Instructors.Where(
            i => i.ID == id.Value).Single().Courses;
    }

    if (courseID != null)
    {
        ViewBag.CourseID = courseID.Value;
        viewModel.Enrollments = viewModel.Courses.Where(
            x => x.CourseID == courseID).Single().Enrollments;
    }

    return View(viewModel);
}

 public class InstructorIndexData
    {
        public IEnumerable<Instructor> Instructors { get; set; }
        public IEnumerable<Course> Courses { get; set; }
        public IEnumerable<Enrollment> Enrollments { get; set; }
    }

我认为这些代码行将加载**所有**讲师以及每个课程相关的OfficeAssignment和**所有**讲师课程以及与每门课程相关的Department.

viewModel.Instructors = db.Instructors
        .Include(i => i.OfficeAssignment)
        .Include(i => i.Courses.Select(c => c.Department))   
        .OrderBy(i => i.LastName);

如果是，为什么之后，如果 Select 了讲师(id！=空)，则再次加载视图模型课程？

viewModel.Courses = viewModel.Instructors.Where(i => i.ID == id.Value).Single().Courses;个

有没有人能解释一下？