Question

Relatively new to MVC and trying to get a cascading dropdown list working for train times.

After looking at a lot of posts, people say that you should stay away from ViewBag/ViewData and instead focus on ViewModels, but I just can’t seem to get my head round it, and it’s driving me up the wall. Any tutorial seems to be either to complex or too easy and the whole viewModel idea just hasn’t clicked with me yet.

So here is my scenario: I have an admin system where staff can add individual train journeys. For each train time, I have an input form where the user can choose a Company, and from there, I’d like the dropdownlist underneath to populate with a list of journey numbers, which indicate routes. Once they have chosen a number, they can carry on with the rest of the form, which is quite large, including time of travel, facilities on the train etc.

I’ve created a viewmodel like so:

public class JourneyNumbersViewModel
    {
        private List<SelectListItem> _operators = new List<SelectListItem>();
        private List<SelectListItem> _journeys= new List<SelectListItem>();

        [Required(ErrorMessage = "Please select an operator")]
        public string SelectedOperator { get; set; }
        [Required(ErrorMessage = "Please select a journey")]
        public string SelectedJourney { get; set; }

        public List<SelectListItem> Journeys
        {
            get { return _journeys; }
        }
        public List<SelectListItem> Operators
        {
            get
            {
                foreach(Operator a in Planner.Repository.OperatorRepository.GetOperatorList())
                {
                    _operators.Add(new SelectListItem() { Text = a.OperatorName, Value = a.OperatorID.ToString() });
                }
                return _operators;
            }
        }
    }

In my controller, I have this for the Create view:

    public ActionResult Create()
    {
        return View(new JourneyNumbersViewModel());
    }

And this is where it isn’t really working for me – if I change my model at the top of the Create view to: @model Planner.ViewModels.JourneyNumbersViewModel, then the rest of my form throws errors as the model is no longer correct for the rest of the form. Is this the way it is supposed to work – what if you need to reference multiple view models with a single view?

I know this is a simple thing and as soon as it clicks I’ll wonder how on earth I could have struggled with it in the first place, but if anyone can point out where I’m going wrong, I’d be very grateful.

Answer 1

I have done something similar. Here is some of the code (apologies upfront for this being quite long, but I wanted to make sure you could re-create this on your side):

View looks like this:

using Cascading.Models
@model CascadingModel


@{
    ViewBag.Title = "Index";
    Layout = "~/Views/Shared/_Layout.cshtml";
}

<h2>Cascading Forms</h2>
<table>

@using(Html.BeginForm("Index", "Home"))
{
<tr>
    <td>@Html.LabelFor(m=>m.CategoryId)</td>
    <td>@Html.DropDownListFor(m => m.CategoryId, new SelectList(Model.Categories, "Id", "Name"), string.Empty)</td>
</tr>
<tr>
    <td>@Html.LabelFor(m=>m.ProductId)</td>
    <td>@Html.CascadingDropDownListFor(m => m.ProductId, new SelectList(Model.Products, "Id", "Name"), string.Empty, null, "CategoryId", "Home/CategorySelected")</td>
</tr>
<tr>
    <td>&nbsp;</td>
    <td><input type="submit" value="Go"/></td>
</tr>
}
</table>

the Model looks as follows:

public class CascadingModel
{
    public int CategoryId { get; set; }
    public List<Category> Categories { get; set; }
    public int ProductId { get; set; }
    public List<Product> Products { get; set; }
}

the real “clever” part of the system is the Html.CascadingDropDownListFor which looks as follows:

public static class MvcHtmlExtensions
{
    public static MvcHtmlString CascadingDropDownListFor<TModel, TProperty>(
        this HtmlHelper<TModel> htmlHelper,
        Expression<Func<TModel, TProperty>> expression,
        IEnumerable<SelectListItem> selectList,
        string optionLabel,
        IDictionary<string, Object> htmlAttributes,
        string parentControlName,
        string childListUrl
        )
    {
        var memberName = GetMemberInfo(expression).Member.Name;

        MvcHtmlString returnHtml = Html.SelectExtensions.DropDownListFor(htmlHelper, expression, selectList, optionLabel, htmlAttributes);

        var returnString = MvcHtmlString.Create(returnHtml.ToString() + 
                    @"<script type=""text/javascript"">
                        $(document).ready(function () {
                            $(""#<<parentControlName>>"").change(function () { 
                                var postData = { <<parentControlName>>: $(""#<<parentControlName>>"").val() };
                                $.post('<<childListUrl>>', postData, function (data) {
                                    var options = """";
                                    $.each(data, function (index) {
                                        options += ""<option value='"" + data[index].Id + ""'>"" + data[index].Name + ""</option>"";
                                    });
                                    $(""#<<memberName>>"").html(options);
                                })
                                .error(function (jqXHR, textStatus, errorThrown) { alert(jqXHR.responseText); });
                            });
                        });
                     </script>"
                    .Replace("<<parentControlName>>", parentControlName)
                    .Replace("<<childListUrl>>", childListUrl)
                    .Replace("<<memberName>>", memberName));

        return returnString;

    }

    private static MemberExpression GetMemberInfo(Expression method)
    {
        LambdaExpression lambda = method as LambdaExpression;
        if (lambda == null)
            throw new ArgumentNullException("method");

        MemberExpression memberExpr = null;

        if (lambda.Body.NodeType == ExpressionType.Convert)
        {
            memberExpr = ((UnaryExpression)lambda.Body).Operand as MemberExpression;
        }
        else if (lambda.Body.NodeType == ExpressionType.MemberAccess)
        {
            memberExpr = lambda.Body as MemberExpression;
        }

        if (memberExpr == null)
            throw new ArgumentException("method");

        return memberExpr;
    }
}

Controller Logic for those looking for it:

public ActionResult CategoriesAndProducts()
{
    var viewModel = new CategoriesAndProductsViewModel();
    viewModel.Categories = FetchCategoriesFromDataBase();
    viewModel.Products = FetchProductsFromDataBase();
    viewModel.CategoryId = viewModel.Categories[0].CategoryId;
    viewModel.ProductId = viewModel.Products.Where(p => p.CategoryId).FirstOrDefault().ProductId;
    return View(viewModel);
}