Selecting a row in a table in ASP.NET MVC 5

Question

I am following Getting Started with Entity Framework 6 Code First using MVC 5 tutorial. In this tutorial, in a view there are two tables. When you select a row from first (Instructor) table, the below table displays that Instructor's courses. I can not understand how is this happening. Where in the code does it know that instructor's courses? If someone could explain this to me, I'd be very glad. Thanks in advance. Here is the controller :

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);
        }

Here is the view :

@model ContosoUniversity.ViewModels.InstructorIndexData
@{
    ViewBag.Title = "Instructors";
}

<h2>Instructors</h2>

<p>
    @Html.ActionLink("Create New", "Create")
</p>
<table class="table">
    <tr>
        <th>
            Last Name
        </th>
        <th>
            First Name
        </th>
        <th>
            Hire Date
        </th>
        <th>
            Office
        </th>
        <th></th>
    </tr>

    @foreach (var item in Model.Instructors)
    {

        string selectedRow = "";
        if (item.ID == ViewBag.InstructorID)
        {
            selectedRow = "success";
        }

        <tr class="@selectedRow">
            <td>
                @Html.DisplayFor(modelItem => item.LastName)
            </td>
            <td>
                @Html.DisplayFor(modelItem => item.FirstMidName)
            </td>
            <td>
                @Html.DisplayFor(modelItem => item.HireDate)
            </td>
            <td>
                @if (item.OfficeAssignment != null)
                {
                    @item.OfficeAssignment.Location
                }
            </td>
            <td>
                @Html.ActionLink("Select", "Index", new { id = item.ID }) |
                @Html.ActionLink("Edit", "Edit", new { id = item.ID }) |
                @Html.ActionLink("Details", "Details", new { id = item.ID }) |
                @Html.ActionLink("Delete", "Delete", new { id = item.ID })
            </td>
        </tr>
    }

</table>


@if (Model.Courses != null)
{
    <h3>Courses Taught by Selected Instructor</h3>
    <table class="table">
        <tr>
            <th></th>
            <th>Number</th>
            <th>Title</th>
            <th>Department</th>
        </tr>
        @foreach (var item in Model.Courses)
        {
            string selectedRow = "";
            if (item.CourseID == ViewBag.CourseID)
            {
                selectedRow = "success";
            }
            <tr class="@selectedRow">
                <td>
                    @Html.ActionLink("Select", "Index", new { courseID = item.CourseID })
                </td>
                <td>
                    @item.CourseID
                </td>
                <td>
                    @item.Title
                </td>
                <td>
                    @item.Department.Name
                </td>
            </tr>
        }
    </table>
}

Answer 1

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

This is the code that selects the courses based on the instructor. Pay attention to the fact that it first selects the Instructor based on the id parameter and the reads the Courses of that Instructor

Answer 2

i think the code is clear, in index action it finds the Instructor with given id, this block:

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

everything is done by ViewBags , ViewBag.InstructorID defines selected

'Instructor' , this block in View page

if (item.ID == ViewBag.InstructorID)
{
 selectedRow = "success";
}

'tr tag' that has "success" class is selected

same logic for courses