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up vote 1 down vote favorite

I have a table with several rows of data that I need to return to the controller. In my view, I initially load the table by selection of a time period and clicking a button. The table loads all my relevant records but one of my table cells contains a dropdown list. So I should be able make a selection in the dropdown click "update" and my controller saves the changes.

So everything works until I try and save. The model that is sent to the controller is completely null. The list property I have tied to the table cells returns to the controller null.

 @ModelType SuperViewModel
 //We need this a view model in order to store a List of the models in the table   

 @Using (Html.BeginForm())
 @For Each i in Model.CompleteList
    Dim currentItem = i //MVC auto-generated extra declarations. Seems redundant to me but it works.
 @<table>
 <tr>
 <td>@Html.DisplayFor(function(Model)currentItem.Name)</td>
 <td>@Html.DisplayFor(function(Model)currentItem.SampleTime)</td>
 <td>@Html.DropDownListFor(function(Model)currentItem.WorkTime, ViewBag.WorkTimeList)</td>
 </tr>
 Next
 </table>
 <input name="submit" type="submit" value="Update"/>
 End Using

 //Controller
 <HttpPost()>
 function Save(vmodel as SuperViewModel, submit as String) as ActionResult //NOTE: submit parameter is used because we have two submit buttons but its not relevant here
      if submit = "Update"
            db.Entry(vmodel.CompleteList).State = EntityState.Modified//Here the exception is throw because our list is null at this point even tho its tied to the model in the view.
            db.SaveChanges()
      end if
 End Function

NOTE: This is written in VB.NET but C# help is welcome. I am familiar with both languages in MVC.

share|improve this question
    
How is SuperViewModel defined? –  Erik Funkenbusch Apr 23 '12 at 23:03
    
There's several properties in it but the one that applies here is the "CompleteList" which is a List(Of AnotherModel). Which that other model was generated by Entity Framework based of our DB table. –  ExceptionLimeCat Apr 23 '12 at 23:08
    
Is there a Next missing before End Using to close the For Each? Also, can you add some of the markup rendered by the view? –  Brian Cauthon Apr 23 '12 at 23:33
    
no the next is there –  ExceptionLimeCat Apr 24 '12 at 1:28

3 Answers 3

up vote 1 down vote accepted

you need to use a for iterator and have the view use the indexed element for the HTML element. I don't know the VB.NET syntax, c# example is below. This allows the model binder to correctly determine the element in the view model so that it can repopulate the view model on postback.

<table>
 @For(var i = 0; i < Model.CompleteList.Count; i++) 
{
 <tr>
 <td>@Html.DisplayFor(Model.CompleteList[i].Name)</td>
 <td>@Html.DisplayFor(Model.CompleteList[i]..SampleTime)</td>
 <td>@Html.DropDownListFor(Model.CompleteList[i]..WorkTime, ViewBag.WorkTimeList)</td>
 </tr>
}
 </table>

Your post method should then work fine.

share|improve this answer
    
This is correct. We were not binding our rows to specific instances in our list. Thanks. –  ExceptionLimeCat Apr 26 '12 at 19:05
up vote 0 down vote

It might be returning null due to some conflicts in the parameter name (the parameter names used must be used some where else as well). Try this

<input name="subButton" type="submit" value="Update"/>

and change

function Save(vm as SuperViewModel, subButton as String) as ActionResult
share|improve this answer
    
That didn't work. The SuperViewModel is still null when it hits the controller. –  ExceptionLimeCat Apr 23 '12 at 23:30
    
Refer to this [blog post] (codeblog.shawson.co.uk/…) –  Prashanth Thurairatnam Apr 23 '12 at 23:31
    
That blog didn't really help. I added the ModelState Validation. It passes that if but the view model is still null. –  ExceptionLimeCat Apr 23 '12 at 23:46
up vote 0 down vote

I would need to see the names in the markup generated by the DropDownListFor, but I suspect it is because with currentItem you are binding against the CompleteList type, but your controller method is expecting the full SuperViewModel type. There is nowhere in the code you have shown that I see you binding to the SuperViewModel type.

Try changing your controller method to:

<HttpPost()>
 function Save(currentItem as CompleteList, submit as String) as ActionResult //NOTE: submit parameter is used because we have two submit buttons but its not relevant here
      if submit = "Update"
            db.Entry(vmodel).State = EntityState.Modified//Here the exception is throw because our list is null at this point even tho its tied to the model in the view.
            db.SaveChanges()
      end if
 End Function

The WorkTime property should be populated with the selected value from the DropDownList.

Edit: changed the parameter name to match the binding name.

share|improve this answer
    
<select name="$VB$Local_currentItem.AdjustedTime"> This is the tag for the dropdown list –  ExceptionLimeCat Apr 24 '12 at 0:00
    
<select name="$VB$Local_currentItem.AdjustedTime"> This is the tag for the dropdown list. Are you saying the second declaration below the For Each is causing MVC to not recognize the binding to the model? –  ExceptionLimeCat Apr 24 '12 at 0:05
    
Interesting. I'm not familiar with VB version of model binding, but the reason you are getting Nothing (or nulls) is because the model binder can't map the values to your controller parameters. It looks to me like the model expects to map to something called currentItem As CompleteList. –  mgnoonan Apr 24 '12 at 0:06
    
Yes, the naming is very important. The model binding will try to map things with the same name as long as they can be cast to the requested type. –  mgnoonan Apr 24 '12 at 0:10
    
Yea I just commented out the second dim and changed all my displayfor/hiddenfor to @Html.HiddenFor(Function(Model) i.SequenceNumber)@Html.DisplayFor(Function(Model) i.SequenceNumber) With no success. Still receiving nulls(Nothings) –  ExceptionLimeCat Apr 24 '12 at 0:13

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