Creating the ViewModel:

The first step is to create a ViewModel which will represents the elements on the View. Our ViewModel revolves around the relationship between the category and products. The user will select a category from a dropdownlist which will populate all the corresponding products in another dropdownlist. The implementation bellows shows the ArticleEditViewModel:

using System;
using System.Collections.Generic;
using System.ComponentModel.DataAnnotations;
using System.Linq;
using System.Web;
using System.Web.Mvc;
namespace MvcApplication1.ViewModels
    public class ArticleEditViewModel
        private List<SelectListItem> _categories = new List<SelectListItem>();
        private List<SelectListItem> _products = new List<SelectListItem>();
        [Required(ErrorMessage = "Please select a category")]
        public string SelectedCategory { get; set; }
        [Required(ErrorMessage = "Please select a product")]
        public string SelectedProduct { get; set; }
        public List<SelectListItem> Products
            get { return _products;  }
        public List<SelectListItem> Categories
                _categories.Add(new SelectListItem(){ Text = "Ruby", Value = "1"});
                _categories.Add(new SelectListItem(){ Text = "JavaScript",Value="2"});
                return _categories;

As, you can see that to keep the demo simple we have hard-coded the categories. In the next section we will demonstrate how to populate the categories dropdownlist using the ArticleEditViewModel.

Populating Categories DropDownList:

The Index action of the HomeController returns the new ArticleEditViewModel to the view. The view uses the Categories property of the ArticleEditViewModel to populate the dropdownlist. The Index action is shown below:

public ActionResult Index()

           return View(new ArticleEditViewModel());

The DropDownList is populated using the code below:

<text>Select a category: </text> @Html.DropDownListFor(x => x.SelectedCategory, new SelectList(Model.Categories,"Value","Text"),"Please Select a category") 
@Html.ValidationMessageFor(x => x.SelectedCategory) 

The DropDownList is bind to the SelectedCategory property of the ViewModel. The SelectedCategory property is also decorated with the [Required] attribute which makes sure that the SelectedCategory is not left blank or on the default item. The screenshot below shows the validation being fired when the Categories DropDownlist is left at the default option and the view is submitted.

The validation is fired because the default option "Select a category" does not have any value associated with it. ASP.NET MVC framework tries to bind the empty string with the SelectedCategory property but fails since the SelectedCategory is marked with [Required] attribute.  
Fetching the Products and Populating the Dependent DropDownList:

Our next task is to get the selected category and then fetch the associated products using the categoryId. This will be performed by an ajax call using our beloved jQuery library. The implementation below shows the details.

<script language="javascript" type="text/javascript">
    $(document).ready(function () {
        $("#SelectedCategory").change(function () {
            $.get('/Home/GetProductsForCategory/' + $(this).val(), function (response) {
                var products = $.evalJSON(response);
                var ddlSelectedProduct = $("#SelectedProduct");
                // clear all previous options 
                $("#SelectedProduct > option").remove(); 
                // populate the products 
                for (i = 0; i < products.length; i++) {
                    ddlSelectedProduct.append($("<option />").val(products[i].Value).text(products[i].Text));

The code above calls the GetProductsForCategory action with the categoryId as the input parameter. The GetProductsForCategory returns the JSON response which represents the products. Before populating the DropDownList with the products we clear all the items in the DropDownList. The GetProductsForCategory action is implemented below:

 public string GetProductsForCategory(string id)
           // get the products from the repository 
            var products = new List<SelectListItem>();
            if (id == "1")
                products.Add(new SelectListItem() {Text = "Introduction to Ruby", Value = "1"});
                products.Add(new SelectListItem() {Text = "Ruby Unit Testing", Value = "2"});
            else if(id == "2")
                products.Add(new SelectListItem(){ Text = "JavaScript testing",Value = "1"});
                products.Add(new SelectListItem() { Text = "JavaScript Ninja", Value = "2"});
            return new JavaScriptSerializer().Serialize(products); 

The above action simply returns the hard-coded values but in actual application you will retrieve the products from a data storage system.

The screenshot below shows the final result:

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