Nullable Code

Alot of developers who are only getting started with either jQuery and/or MVC wants to know how to create Cascading Dropdowns, and the infamous Auto-Complete function for textboxes.

In this article, I will describe how you too can achieve this with a few simple steps.

Cascade Dropdowns

So lets begin with Cascading Dropdowns.

You have a primary dropdown, and then based on the selection of the primary dropdown item, you want to populate the secondary dropdown accordingly. This can be achieved using MVC methods in combination with jQuery calls.

In the sample above, I have a dropdown called “Call Type”. Based on the Call Type selected, the Category list is then automatically filtered for user input.

Below is the cshtml snippet for the control UI:

<div class="span3">
     @Html.Label("Call Type", new { @class = "label-bold" })
     @Html.DropDownList("ViewerCallType", String.Empty)
</div>
<div class="span3">
     @Html.Label("Category", new { @class = "label-bold" })
     @Html.DropDownList("CommentCategory", String.Empty)
</div>

To handle the select event of the Primary Dropdown, add a delegate to the ‘onchange’ event as below:

            $("#ViewerCallType").change(function (e) {
                $.getJSON("/Home/GetCommentCategory", {
                    'ViewerCallType': $("#ViewerCallType > option:selected").attr("value")
                }, function (data) {
                        var items = "---------------------";
                        $.each(data, function (i, category) {
                            items += "
" + category.text + "
";
                        });
                        $("#CommentCategory").html(items);
                    });
            });

The code above will call the “GetcommentCategory” action in the Home controller class, and pass in the selected CallerType value. On return, the JSON data is traversed. The option tags are generated, and then the list is assigned to the CommentCategory control. Next let’s take a look at the controller.

The Controller

        public JsonResult GetCommentCategory(string ViewerCallType)
        {
            var list = (from i in db.MyCategoryTable
                        where i.CallType.Equals(ViewerCallType, StringComparison.CurrentCultureIgnoreCase)
                        select new
                        {
                            value = i.MyValue,
                            text = i.MyText
                        }).OrderBy(i => i.value);

            return Json(list, JsonRequestBehavior.AllowGet);
        }

Pretty easy right? Using Linq, make a query to the data table, filtering by the selected Call Type value that is passed in as the first parameter. On exit, simply return the list in the JSON format by using the Json() method.

Auto-Completes

Adding Autocomplete to a text is just as easy!

So say you have the following textbox to display a list of email recipients.
This can be a BIG list, so it’s nice to filter the list.

Firstly, let’s create the textbox:

                @Html.Label("Recipient", new { @class = "label-bold" })
                @Html.TextBox("Recipient", Model.Recipient)

Then.. we can add the jQuery code to turn this basic input control into a jQuery auto-complete control:

            $('#Recipient).autocomplete(
                {
                    source: "/Home/GetRecipients",
                    minLength: 3,
                    select: function (event, ui) {
                        $('#Recipient).val(ui.item.value);
                    }
             });

You will need to download and link the autocomplete control JS files as appropriate: http://jqueryui.com/autocomplete/

The above code tells jQuery to execute the action “GetRecipients” in the Home controller, only when there are 3 characters or more entered into the text box (this is restrict network traffic and prevent overloads). On select of an item, the value of the selected item will be assigned to the auto-complete textbox.

Here’s the controller method:

        public JsonResult GetRecipients(string term)
        {
            var list = (from i in db.Employees
                        where ((i.FirstName.StartsWith(term) || i.LastName.StartsWith(term))))
                        select new
                        {
                            value = i.DisplayName, 
                            id = i.EmailAddress
                        }).OrderBy(i => i.value).Take(20);

            return Json(list, JsonRequestBehavior.AllowGet);

        }

There you have it. Hope this helped!