MVC Series Part 1: Dynamically Adding Items Part 3

Collections named with same substring in a Dynamic Item

Click here to download code and sample project

In the second part of the series, I discussed how to include a collection inside a dynamic item.┬áHere is one gotcha that I ran into during development. Let’s say we decide to add a Publisher for every new book and you want the ability to add a few books the publisher has distributed. Here is the PublisherViewModel class I created:

public class PublisherViewModel
	{
		public PublisherViewModel()
		{
			Books = new List<PublisherBookViewModel>();
			for ( int i = 0; i < 3; i++ )
			{
				Books.Add( new PublisherBookViewModel() );
			}
		}
		public string Name { get; set; }

		public IList<PublisherBookViewModel> Books { get; set; }
	}

Here is how the html would look for PublisherViewModel:

<dl class="dl-horizontal">
    <dt>
        @Html.DisplayNameFor( model => @Model.Name )
    </dt>
    <dd>
        @Html.EditorFor( model => @Model.Name )
    </dd>
    <br />

    <dt>
        @Html.DisplayNameFor( model => @Model.Books )
    </dt>
    <dd>
        @for ( int i = 0; i < @Model.Books.Count(); i++ )
        {
            @Html.EditorFor( model => @Model.Books[i] )
        }
    </dd>
</dl>

I went ahead and created a new class for the books collection called PublisherBookViewModel. It simply contains one property for Name. Here is how the Html looks:

@using ( Html.BeginCollectionItem( "Books" ) )
{
    <dl class="dl-horizontal">
        <dt>
            @Html.DisplayNameFor( model => @Model.Name )
        </dt>
        <dd>
            @Html.EditorFor( model => @Model.Name )
        </dd>
    </dl>
}

But when I go to create one new Book and post back, I actually end up receiving 4 items in my ‘NewBooks’ collection. How did this even happen?

Well, the reason this occurred is again thanks to our BeginCollectionItem HtmlHelper class. The extra logic that we added in the last article seems to merely grab the first occurrence of our collection name:

collectionName = htmlFieldPrefix.Substring( 0, htmlFieldPrefix.IndexOf( collectionName ) + collectionName.Length );

If we look at what the HtmlFieldPrefix string looks like coming in we can start to spot the problem:

"NewBooks[18fe8281-9b41-43ff-bcc9-a14ce3e99dc8].Publisher.Books[0]"

So, our parent dynamic item is being bounded to a collection called ‘NewBooks’, but the PublisherViewViewModel has its own collection of items called ‘Books’. Our Substring method needs to be a little bit smarter by replacing ‘IndexOf’ with ‘LastIndexOf’. And with that, we will no longer have issues with sub collection binding.

This is as far as I took with dynamically adding items in MVC, but at some point it would be nice to be able to add dynamic items to a dynamic item (i.e., instead of pre-populating our Characters collection, we let the user add as many as they need). Perhaps another day I will delve into this.

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