Losing data in models and collections inside the ViewModel on postback - asp.net-mvc-3

Question

I am using a viewmodel that contains a few other models(entities) for each partial view.
I am rendering a partial view by passing the entity which is inside the ViewModel. My partial view has a few fields and some buttons. On click of button (which is inside my partial view) the form is being posted back with the data in a sub entity, whereas my viewmodel is always posted back as null...
I need the data to be present in my viewmodel on post back.
All views are strongly typed:
Code:
public class OrdersVM
{
public FiltersVM filterCriteria { get; set; }
public IEnumerable<MeterInventory> meters { get; set; }
public string assignTo { get; set; }
public IEnumerable<SelectListItem> AssigneeOptions { get; set; }
}
public partial class Meters
{
public int MTRNO { get; set; }
public string LOCName { get; set; }
}
public class FiltersVM
{
public string Center { get; set; }
public DateTime? DueDate { get; set; }
}
View Code
#model OrdersVM
#{
ViewBag.Title = "Orders";
}
#using (Html.BeginForm())
{
<div>
#Html.Partial("~/Views/Base/Filters.cshtml", Model.filterCriteria)
</div>
#foreach (var item in Model.meters)
{
<table>
<tr>
<td>
#Html.Encode(item.LOCNAME)
</td>
</tr>
</table>
}
}
Controller code
[HttpPost]
public ActionResult Index(OrdersVM orders, FiltersVM filters)
{
//orders is null
//filters has values
}
Thanks Olivehour. I am using the partial view "Filters.cshtml". and am rendering the same.
Below is the code for partial view :
#model ViewModels.FiltersVM <fieldset>
<legend>Order Assignment</legend>
<table id="tbl1" class="tableforcontrols">
<tr>
<td>
<div class="editor-label">
#Html.LabelFor(model => model.LDC)
</div>
</td>
<td>
<div class="editor-field">
<input type="submit" value="Search" id="btnSearch" name="button" />
</div>
</td>
<td>
<div class="editor-field">
<input type="submit" class="cancel" value="Reset" id="btnReset" name="button" />
</div>
</td>
</tr>
</table> </fieldset>
I tried with single argument "OrdersVM" (parent view model) but no luck.
[HttpPost]
public ActionResult Index(OrdersVM orders)
but if I pass the parent viewmodel to the partial view it was holding the data in OrdersVM.filterCriteria but not for properties (IEnumerable meters, string assignTo and Enumerable AssigneeOptions)
#Html.Partial("~/Views/Base/Filters.cshtml", Model)
I am new to MVC. Please let me know if any one finds the solution.
Thanks in advance.

Answer 1

It looks like you have a couple of problems here. One probable reason why the orders arg is null in your action method is because it doesn't look like you are rendering any input elements. You just have #Html.Encode(item.LOCNAME).
In order for the default model binder to construct an instance of OrdersVM and pass it to the action method, it needs to have input from the HTTP POST. You need something more like #Html.TextBoxFor(m => item.LOCNAME).
The second problem I think is that you have 2 arguments in the action method. Since the OrdersVM already has a FiltersVM property, you should just be able to have a single OrdersVM argument to the action method. During the HTTP POST, you can just access FiltersVM properties from OrdersVM.filterCriteria. This will lead to your 3rd challenge, though, since the meters property on OrdersVM is an IEnumerable collection.
To solve this, first have a couple reads of this article about model binding collections. It's old, but it still applies in MVC3. Read it and really wrap your head around it.
If you don't like using integers to index your collection fields, there is an HTML helper written by Steve Sanderson that allows you to index collection inputs using GUID's. We use this all the time, but it can be tricky -- mainly, you should always put the collection item in a partial view. For now, you might just be better off using integer-based indexing as outlined in the Haacked article.

Answer 2

It sounds like you are comming from Webforms. To transition to MVC you need to remove the thought of PostBack. This is concept that doesn't really exist on the web but Webforms faked it for us.
In MVC you usually start with a GET request like /edit/{someId}. From here you load the data for the viewmodel from the database and render the view. Now let's say that all data in the viewmodel is editable so each property have it's own input field. The user edits some data and saves the form. This issues a POST to the server.
Assume we have this POST method
[HttpPost]
public ActionResult Edit(MyViewModel model)
In this case you have all the data you need modelbinded because all data existed in the form.
You could do this and get the same view rendered because all data was databinded.
[HttpPost]
public ActionResult Edit(MyViewModel model){
return View(model);
}
Now let's pretend you have a dropdown in your form. Then you would have these two properties in your viewmodel.
public int CarId { get; set; }
public IEnumerable<SelectListItem> CarOptions {get; set; }
When you post the form this time the CarId will be populated in the ViewModel but not CarOptions because they are not a part of the form data. What you do if you would want to return the same view again is to reload the missing parts.
[HttpPost]
public ActionResult Edit(MyViewModel model){
model.CarOptions = LoadCarOptions();
return View(model);
}
It's certainly possible to modelbind that too if you put it in a hidden field. But it's easier and probably more effective to reload it from server/database again. This is the normal approach taken when working with MVC.