ActionMethod param is 0 every time - asp.net-mvc-3

Question

I've got a form (here's some of its code):
#model Discussion
#{
ViewBag.Title = "Edit Discussion";
Layout = "~/Views/Shared/_App.cshtml";
}
#using (Html.BeginForm("Update", "Discussion", FormMethod.Post, new { id = Model.discussionId, #class = "update" }))
{
... rest of the view code
}
My related controller's update method has:
[HttpPost]
[ValidateInput(false)]
public ActionResult Update(FormCollection col, int id = 0)
{
... rest of code
}
when the form is submitted, I keep getting an id of 0 instead of the ID I'm seeing from the model that when I debug is clearly there inside being used during the form rendering. IT's just when I submit, it's not passing the id correctly.
Am I doing something wrong in terms of syntax here? the new{} in the Html.BeginForm I guess I don't understand how the new anonymous type is matching up to this id when the form is submitted when it's a collection of id and a class here as you can see.

Answer 1

When you do the new {id = Model.discussionId ...} you are setting the id attribute of the form element. To get the id to come across properly you would have to make it an input of the form. So inside the form you would put a hidden input element with something like this: #Html.Hidden("id", Model.discussionId)
So your current form is rendering something like this:
<form id="theDiscussionId" class="update" action="/Discussion/Update">
... rest of the view code
</form>
And you need it to be like this:
<form class="update" action="/Discussion/Update">
<input type="hidden" name="id" value="theDiscussionId" />
... rest of the view code
</form>