Why does the action method get wrong parameter?

Question

I have the Action method and the View for editing properties of some items.

    [HttpPost]
    [ValidateAntiForgeryToken]
    [Authorize(Roles = "Admin")]
    public async Task<ActionResult> Edit(Item item)
    {
        if (ModelState.IsValid)
        {
            db.Entry(item).State = EntityState.Modified;

            await db.SaveChangesAsync();
            return RedirectToAction("Index");
        }
        ViewBag.CatagorieId = new SelectList(db.Catagories, "ID", "Name", item.CatagorieId);
        return View(item);
    }

and

@model OpenOrderFramework.Models.Item
@using OpenOrderFramework.Extensions
@{
    ViewBag.Title = "edit";
}

<h2>Editing</h2>


@using (Html.BeginForm())
{

    <div class="form-horizontal">
        <h4>The car</h4>
        <hr />
        @Html.ValidationSummary(true, "", new { @class = "text-danger" })
        @Html.HiddenFor(model => model.ID)
         <-- etc -->

But when I submit the form I get an error

Store update, insert, or delete statement affected an unexpected number of rows (0).

I figured out that in action method ID of the item that was posted is always 0 even if real ID of the item is different. Why does it happen?

GET Action method:

    // GET: Items/Edit/5
     [Authorize(Roles = "Admin")]
    public async Task<ActionResult> Edit(int? id)
    {
        if (id == null)
        {
            return new HttpStatusCodeResult(HttpStatusCode.BadRequest);
        }
        Item item = await db.Items.FindAsync(id);
        if (item == null)
        {
            return HttpNotFound();
        }
        ViewBag.CatagorieId = new SelectList(db.Catagories, "ID", "Name", item.CatagorieId);
        return View(item);
    }

`item` won't be tracked by Entity Framework because it originated from the POST. — , Dec 29 '15 at 21:17
Use a view model to pass data between your view and action method. http://stackoverflow.com/a/34260397/40521 — Shyju, Dec 29 '15 at 21:18
It's a terrifyingly bad practice to use your EF models as parameters to your controller actions. Not to mention how terrifyingly bad practice is to use those same EF objects as models in your ASP.NET MVC views. The solution to your problem is of course to use view models. — Darin Dimitrov, Dec 29 '15 at 21:19
Ok, thanks. If you write it as answer of the question I'll check it as accepted. — Vitalii Vitrenko, Dec 29 '15 at 21:24
Yes, your code is completely wrong because you are not using a view model. — Darin Dimitrov, Dec 29 '15 at 21:28

Shyju · Accepted Answer · 2015-12-29T21:36:21.843

When you post the form, the http call to your HttpPost action method is a totally separate Http request and Entity framework cannot track that entity.

As Darin mentioned in the comment, It is not a good idea to mix the entity classes in your UI layer. That makes it very tightly coupled.

What you should be using is creating and using a view model for your view. View model's are simply POCO classes, which is specific to the view.

public class ItemViewModel
{
  public int Id {set;get;}
  public string Name {set;get;}
  public List<SelectListItem> Categories { set;get;}
  public int SelectedCategory {set;get;}
}

And in your GET action, read the entity from your database,create an object of the view model and set the property values to that

public ActionResult Edit(int id)
{
   var vm=new ItemViewModel { Id=id };
   var item = db.Items.FirstOrDefault(s=>s.Id==id);
   if(item!=null)
   {
     vm.Name = item.Name;
   }
   vm.Categories =db.Categories.Select(s=> new SelectListItem { Value=s.Id.ToString(),
                               Text=s.Name 
                            }).ToList();
   return View(vm);
}

And your view will be strongly typed to your view model

@model ItemViewModel
@using(Html.BeginForm())
{
  @Html.DropdDownListFor(s=>s.SelectedCategory,Model.Categories,"Select")
  @Html.HiddenFor(s=>s.Id)
  @Html.TextBoxFor(s=>s.Name)

  <input type="submit" />
}

And in your HttpPost action, read the existing entity from your db and update the property values you want to update.

[HttpPost]
public ActionResult Edit(ItemViewModel model)
{      
  if(ModelState.IsValid)
  {
     var item = d.Items.FirstOrDefault(s=>s.Id==model.Id);
     item.Name = model.Name;
     db.Entry(item).State = EntityState.Modified;
     db.SaveChanges();
     return RedirectToAction("Index");
  }
  model.Categories =db.Categories.Select(s=> 
          new SelectListItem { 
                               Value=s.Id.ToString(),
                               Text=s.Name }).ToList();
  return View(model);
}

Make sure to add enough NULL checkings before accessing the entities/ objects in the code.

Is there reason why ModelState.IsValid always return false in this case? — Vitalii Vitrenko, Dec 30 '15 at 15:45
Because you did not set submit some value properly. Take a look at [this](http://stackoverflow.com/a/15296109/40521) answer to see which one is failing — Shyju, Dec 30 '15 at 15:48

Why does the action method get wrong parameter?

1 Answers1