Passing a selected value from a partial view to the main view's viewmodel

As ASP.Net MVC3 newbies we have an issue that would like assist开发者_运维问答ance with. (Questions at the bottom of this thread too.)

First of all I am not sure if the following is the best way to go about this so please let me know if we are heading in the wrong direction. We would like to use partial views to do lookups for dropdown lists. In some cases the lookups will be done in multiple places and also, the data is not part of our viewmodel. The data may be coming from a Database or Web Service in our application. Some of the data is loaded at startup and some is based upon other values selected in the form.

We are calling a child action from our main view and returning a partial view with the data we obtained. Once the user selects their choice we are not sure how to store the selected item code in our main view model.

In our main form we call to an action:

@model Apps.Model.ViewModels.AVMApplicationInfo
        ...
        <div class="editor-label">
            @Html.LabelFor(m => m.VMResidencyWTCS.DisplayState) 
        </div>
        <div class="editor-field">
            @Html.TextBoxFor(m => m.VMResidencyWTCS.DisplayState)
            @Html.DropDownListFor(m => m.VMResidencyWTCS.DisplayState, Apps.Model.Helpers.ResidencyStates.StateList)
            @Html.ValidationMessageFor(m => m.VMResidencyWTCS.DisplayState)
        </div>
        @Html.Action("DisplayCounties", "PersonalInfo")
        ...

In the PersonalInfo controller:

    [ChildActionOnly]
    public ActionResult DisplayCounties()
    {
        IList<County> countiesDB = _db.Counties
            .OrderBy(r => r.CountyDescr)
            .Where(r => r.State == "WI"
             && r.Country == "USA")
            .ToList();

        //Create an instance of the county partial view model
        VMCounty countyView = new VMCounty();

        //Assign the available counties to the view model
        countyView.AvailableCounties = new SelectList(countiesDB, "CountyCd", "CountyDescr");

        return PartialView("_DisplayCounties", countyView);
    }

In the _DisplayCounties partial view:

@model Apps.Model.ViewModels.VMCounty 
<div class="editor-label"> 
    @Html.LabelFor(m => m.CountyDescr) 
</div> 
<div class="editor-field"> 
    @Html.DropDownListFor(x => x.SelectedCountyCd, Model.AvailableCounties) 
</div>

How do I assign the SelectedCountyCd to a field in the main form view model (Apps.Model.ViewModels.AVMApplicationInfo )? Are there any issues of when the child action/partial view is called; i.e., is it loaded at start up and can this method be used to include a user choice as a filter for the lookup? If so, how could the value be passed to the child controller; viewbag?

You could pass it as parameter to the child action:

@model Apps.Model.ViewModels.AVMApplicationInfo
...
@Html.Action("DisplayCounties", "PersonalInfo", new { 
    selectedCountyCd = Model.CountyCd // or whatever the property is called
})

and then have the child action take this value as parameter:

[ChildActionOnly]
public ActionResult DisplayCounties(string selectedCountyCd)
{
    IList<County> countiesDB = _db.Counties
        .OrderBy(r => r.CountyDescr)
        .Where(r => r.State == "WI"
         && r.Country == "USA")
        .ToList();

    //Create an instance of the county partial view model
    VMCounty countyView = new VMCounty();

    //Assign the available counties to the view model
    countyView.AvailableCounties = new SelectList(countiesDB, "CountyCd", "CountyDescr");

    // assign the selected value to the one passed as parameter from the main view
    countyView.SelectedCountyCd = selectedCountyCd;

    return PartialView("_DisplayCounties", countyView);
}