Duplicates on runtime MVC4 Entity Framework - asp.net-mvc-4

Question

I get this strange "error". When i run my app (edit action i.e update) i get duplicate values stored in my dB. If i use the debugger, step by step, its working (no duplicates)...
It's a Many-to-Many Relationship so don't bother the Courses NULL value in the images, just trying to figure things out...
All Help is appreciated!
[HttpPost]
public ActionResult Edit(CourseStudentViewModel model)
{
var course = db.Courses
.Where(c => c.Id == model.CourseId)
.Single();
if (ModelState.IsValid)
{
course.Name = model.CourseName;
course.Description = model.CourseDescription;
course.Students = model.Students;
if(course.Id != 0) {
db.Entry(course).State = System.Data.EntityState.Modified;
}
else {
db.Courses.Add(course);
}
db.SaveChanges();
return RedirectToAction("Index");
}
//modelstate not valid, display form
return View(model);
}
I get my viewModel back. All good.
My Old values from dB. I want to update this data. So everything is still good...
My old values are updated to my new Values. Great!
Ok everything works great IF I step with debugger like this. But if i run the app i will get duplicates.... Anyone?
New content:
My Edit-view
#model ValueInjecter.Web.Models.CourseStudentViewModel
#{
ViewBag.Title = "Edit"; }
Edit Course
#using (Html.BeginForm()) {
#Html.HiddenFor(c => Model.CourseId)
#Html.LabelFor(c => Model.CourseName)
#Html.EditorFor(c => Model.CourseName)
#Html.LabelFor(c => Model.CourseDescription)
#Html.EditorFor(c => Model.CourseDescription)
</div>
<hr />
<h2>Students</h2>
<div class="editor-field">
#for (int i = 0; i < Model.Students.Count(); i++)
{
<div style="border: dotted 1px; padding: 5px; margin: 10px;">
#Html.HiddenFor(s => s.Students[i].Id)
#Html.LabelFor(s => s.Students[i].Name[i + 1])
#Html.EditorFor(s => s.Students[i].Name)
</div>
}
</div>
<p>
Number of Students:
<b>#Html.DisplayFor(s => Model.StudentCount)</b>
</p>
<hr />
<p>
<input type="submit" value="Save" />
</p> }

Answer 1

It probably works in the debugger because by inspecting course.Students collection in a property watch window you trigger actually a second database query (after the query that loads the course) due to lazy loading of the Students collection. (Your Course.Students collection is most likely declared as virtual.) If you run without debugger no lazy loading occurs and course.Students stays empty.
You can force that the course.Students collection is always loaded by using eager loading instead of lazy loading (which also saves the second database roundtrip):
var course = db.Courses
.Include(c => c.Students)
.Where(c => c.Id == model.CourseId)
.Single();
Honestly I have no clue why your code works correctly with the loaded collection (in the debugger) and why it works at all. Assigning a complete detached collection like this: course.Students = model.Students, and then just setting the state of the parent to Modified is normally not enough to update a child collection.
But I see "ValueInjecter" in your screenshots. Maybe there is some automatic mapping magic happening that does (accidentally?) the right thing to get a working update.

Answer 2

Ok I finally figured out the solution of my problem. Of course much easier than first thought.
[HttpPost]
public ActionResult Edit(CourseStudentViewModel model)
{
if (ModelState.IsValid)
{
var course = db.Courses.Find(model.CourseId);
course.Name = model.CourseName;
course.Description = model.CourseDescription;
if(model.Students != null)
{
foreach (var item in model.Students)
{
db.Entry(item).State = System.Data.EntityState.Modified;
}
}
if(course.Id != 0) {
db.Entry(course).State = System.Data.EntityState.Modified;
}
else {
db.Courses.Add(course);
}
db.SaveChanges();
return RedirectToAction("Index");
}
//modelstate not valid, display form
return View(model);
}
Still using the view ive pasted above.