View Models and Check Boxes in View - asp.net-mvc

Question

I have a newbie question, which I have tried to understand for the past few days. Hopefully someone can be kind enough to help me understand the programming flow.
Assuming I have a model, the information is stored in the database:
public class Student
{
public int studentID { get; set; }
public string studentName { get; set; }
public strin studentGrade {get; set; }
}
public class StudentDBContext : DbContext
{
public DbSet<Student> Students { get; set; }
}
and I want to display it into the view, with additional checkbox so I can select which students to be promoted into the next grade. I read that one way to do it is by putting into view model:
public class StudentViewModel
{
public bool promoted { get; set; }
public Student stu { get; set; }
}
But I am stuck on is this the way to do it? and if yes, how do you put into the view where it will display all the students with a checkbox next to it. Afterwards, I want to update all the grade for the students whose checkboxes are ticked. For example:
Student A, Student B, Student D promoted from Grade 1 to Grade 2. So I want to display the students, tick Student A, B and D and submit to update the Grade.
Step by step example will be much appreciated.
Update 1:
Controller:
[HttpGet]
public ViewResult CheckBox()
{
var studentViewModels = db.Students.Select(m => new StudentViewModel()
{
stu = m
}).ToList();
return View(studentViewModels);
}
[HttpPost]
public ActionResult CheckBox(IList<studentViewModel> list)
{
foreach (var stuUpdate in list.Where(m => m.promoted))
{
var stuRow = db.Students.Find(stuUpdate.stu.studentID);
stuRow.studentName = stuRow.studentName + "1";
db.Entry(stuRow).State = EntityState.Modified;
db.SaveChanges();
return RedirectToAction("CheckBox");
}
return RedirectToAction("CheckBox");
}
View:
#model IList<School.ViewModels.StudentViewModel>
#using (Html.BeginForm())
{
<table>
<tr>
<th>
</th>
<th>
student ID
</th>
<th>
student name
</th>
<th>
student grade
</th>
</tr>
#foreach (var item in Model) {
<tr>
<td>
#Html.CheckBoxFor(modelItem => item.promoted)
#Html.HiddenFor(modelItem => item.stu.studentID)
</td>
<td>
#Html.DisplayFor(modelItem => item.stu.studentID)
</td>
<td>
#Html.DisplayFor(modelItem => item.stu.studentName)
</td>
<td>
#Html.DisplayFor(modelItem => item.stu.studentGrade)
</td>
</tr>
}
</table>
<input type="submit" value="save" />
}
However currently hit by the following error:
Value cannot be null.
Parameter name: source
Source Error:
foreach (var stuUpdate in list.Where(m => m.promoted))

Answer 1

A very basic "step by step" (done in SO, so I probably did a few mistakes, but you've got the idea).
You have always a few ways to do these kind of things, so... just really take it as a sample, and find other examples to get other ideas.
well, first, in your controller, you will have a GET action (to see the list).
[HttpGet]
public ViewResult StudentList() {
//retrieve all students. With the Select, we create a new instance of StudentViewModel for each student.
//assuming StudentDbContext being a property of your controller, if it's not, you can instantiate a new one (in a using clause)
var studentViewModels = StudentDbContext.Students
.Select(m => new StudentViewModel() {
stu = m
//we don't say nothing about promoted :
//it will be there as "false" by default, which is probably what we want.
}).ToList();
//now we've got a list of StudentViewModel. This will be the model of our view
return View(studentViewModels);
}
Then we've got a view, StudentList.cshtml
in this view, we will display a table, with a line for each student : the studentId (hidden in this case), the name (display only), the grade (display only), and a checkbox.
We need a for loop (not a foreach) to get fine model binding.
#model IList<StudentViewModel>
#using (Html.BeginForm()) {
<table>
<tr>
<th>Student name</th>
<th>Student grade</th>
<th>Promote</th>
</tr>
#for (var i = 0; i < Model.Count(); i++) {
<tr>
<td>#Html.HiddenFor(m => Model[i].Student.studentID)
#Html.DisplayFor(m => Model[i].Student.studentName)
</td>
<td>#Html.DisplayFor(m => Model[i]Student.studentGrade)</td>
<td>#Html.CheckBoxFor(m => Model[i].promoted)</td>
</tr>
}
</table>
<input type="submit" value="save" />
}
This form will lead to another POST action (same name as the GET one, depending of what you have in your Html.BeginForm)
[HttpPost]
public ActionResult StudentList(IList<StudentViewModel> list) {
//we treat only the lines where checkbox has been checked
foreach (var studentViewModel in list.Where(m => m.promoted) {
var student = StudentDBContext.GetById(studentViewModel.Student.studentID);//get student entity instance from context
student.studentGrade ="<new value>";
StudentDBContext.SaveChanges();//save changes, you must have such a method somewhere.
}
return Action("StudentList");
}
Little detail :
Try to respect some really basic "usual" practices : for example in c#, Properties should begin by an uppercase letter (so StudentGrade, StudentName, Promoted, etc).