MVC4: ViewModel is empty on [HttpPost] - asp.net-mvc-4

Question

I'm trying to make a simple news system in MVC4. I'm very new at it, and I have a simple News base class that looks like this:
NewsPost:
public class NewsPost
{
public virtual int Id { get; set; }
public virtual string Subject { get; set; }
public virtual string Content { get; set; }
}
It's then attached to a category class like so:
NewsCategory:
public class NewsCategory
{
public virtual int Id { get; set; }
public virtual string Name { get; set; }
public virtual ICollection<NewsPost> NewsPosts { get; set; }
}
I then have a controller for creating these NewsPosts:
NewsController:
private readonly INewMvcSiteDataSource _db;
public NewsController(INewMvcSiteDataSource db)
{
_db = db;
}
[HttpGet]
public ActionResult Create()
{
var model = new CreateNewsViewModel();
model.Categories = new SelectList(_db.NewsCategories, "Id", "Name");
return View(model);
}
[HttpPost]
public ActionResult Create(CreateNewsViewModel newsModel)
{
if (ModelState.IsValid)
{
int id = int.Parse(newsModel.SelectedCategories.Single(f => f.Selected).Value);
}
return View(newsModel);
}
And lastly, to facilitate with creating the NewsPost, I use a CreateNewsViewModel, like so:
CreateNewsViewModel:
public class CreateNewsViewModel
{
[Required]
public string Subject { get; set; }
[Required]
public string Content { get; set; }
public SelectList Categories { get; set; }
public SelectList SelectedCategories { get; set; }
}
My view looks like this:
Create.cshtml:
#model NewMvcSite.Models.CreateNewsViewModel
#{
ViewBag.Title = "Create";
}
<h2>Create</h2>
#using (Html.BeginForm()) {
#Html.ValidationSummary(true)
<fieldset>
<legend>CreateNewsViewModel</legend>
<div class="editor-label">
#Html.LabelFor(model => model.Subject)
</div>
<div class="editor-field">
#Html.EditorFor(model => model.Subject)
#Html.ValidationMessageFor(model => model.Subject)
</div>
<div class="editor-label">
#Html.LabelFor(model => model.Content)
</div>
<div class="editor-field">
#Html.EditorFor(model => model.Content)
#Html.ValidationMessageFor(model => model.Content)
</div>
<div class="editor-label">
#Html.LabelFor(model => model.Categories)
</div>
<div class="editor-field">
#Html.DropDownListFor(model => model.Categories, Model.SelectedCategories)
</div>
<p>
<input type="submit" value="Create" />
</p>
</fieldset>
}
#section Scripts {
#Scripts.Render("~/bundles/jqueryval")
}
For some reason, my viewmodel isn't returned when I click the submit button to create the newspost, and since there is no parameterless constructor for [HttpPost] Create action, it fails with "No parameterless constructor defined for this object."
I've been trying going through other posts here stating the same problem, but I fail to see the connection between what they are doing and what I am doing. I hope there is someone out there who can help me.
Thank you.

Answer 1

If you want to select category from drop down list, first of all you should add property to your model to hold Id of selected category. Something like this (property Category):
public class CreateNewsViewModel
{
[Required]
public string Subject { get; set; }
[Required]
public string Content { get; set; }
public SelectList Categories { get; set; }
public SelectList SelectedCategories { get; set; }
public int Category { get; set; }
}
after that, you should change code to filling model.Category to this:
model.Categories = new SelectList(categories, "Id", "Name", model.Category);
and than, in your view, editor for Category should look like this:
<div class="editor-field">
#Html.DropDownListFor(m => m.Category, Model.Categories);
</div>

Answer 2

model.Categories = new SelectList(_db.NewsCategories, "Id", "Name");
At this line you fill Categories property
#Html.DropDownListFor(model => model.Categories, Model.SelectedCategories)
Here you trying create select with Categories name and SelectedCategories items
int id = int.Parse(newsModel.SelectedCategories.Single(f => f.Selected).Value);
Here you trying get selected item, BUT on form submit you only sending one number, not list. One of your SelectList must be int

Answer 3

I found the solution.
I had to change the ViewModel to this:
public IEnumerable<SelectListItem> Categories { get; set; }
public int SelectedCategoryId { get; set; }
I changed the controller to:
var model = new CreateNewsViewModel
{
Categories = new SelectList(_db.NewsCategories, "Id", "Name")
};
And changed View to this:
#Html.DropDownListFor(x => x.SelectedCategoryId, Model.Categories)