Ajax model filter on multi DropDownList bind

Question

I have a problem which I'm not sure how to solve (code-wise) and I'd like to get some input from the community.

So, I have a page where I will display product details and these details will vary according to the values selected on a DropDownList (DDL). So, for each product I have a collection of variations of said product which are identified by a UniqueId and by a JSON array of attribute uniqueIds, like this:

[
    {
        "AttributeId": "fe153d69-8ac1-6e0c-8793-ff0000804eb3",
        "AttributeValueId": "64163d69-8ac1-6e0c-8793-ff0000804eb3"
    },
    {
        "AttributeId": "00163d69-8ac1-6e0c-8793-ff0000804eb3",
        "AttributeValueId": "67163d69-8ac1-6e0c-8793-ff0000804eb3"
    }
]

What this tells me is that this particular variation has a Days attribute with a value of 5, a Portions attribute value of 4 and so on..

For this particular product I also have other values/variations with the same scheme. Here are my possible values:

Days | Portions
  3        2
  3        4
  3        6
  5        4

Now, on my View I'm binding two DDLs to each of these attributes (one for days, one for Portions).

Here's my controller Method:

    public ActionResult Details(Guid productId)
    {
        CatalogManager catalogManager = CatalogManager.GetManager();
        EcommerceManager ecommerceManager = EcommerceManager.GetManager();

        ProductListWidgetModel model = new ProductListWidgetModel();
        model.Product = catalogManager.GetProduct(productId);

        //Read the variant properties
        JavaScriptSerializer serializer = new JavaScriptSerializer();

        model.ProductVariations = catalogManager.GetProductVariations(productId).ToList();
        model.Portions = new List<SelectListItem>();
        model.Meals = new List<SelectListItem>();

        foreach (var item in model.ProductVariations)
        {
            List<AttributeValuePair> attributeValuePairs = serializer.Deserialize<List<AttributeValuePair>>(item.Variant);
            foreach (var attribute in attributeValuePairs)
            {
                if (catalogManager.GetProductAttribute(attribute.AttributeId).Title == "Portions")
                {
                    var attributes = catalogManager.GetProductAttributeValue(attribute.AttributeValueId);
                    if (!model.Portions.Exists(x => x.Value == attributes.Title))
                    {
                        model.Portions.Add(new SelectListItem { Value = attributes.Id.ToString(), Text = attributes.Title });
                    }
                }
                if (catalogManager.GetProductAttribute(attribute.AttributeId).Title == "Days")
                {
                    var attributes = catalogManager.GetProductAttributeValue(attribute.AttributeValueId);
                    if (!model.Meals.Exists(x => x.Value == attributes.Title))
                    {
                        model.Meals.Add(new SelectListItem { Value = attributes.Id.ToString(), Text = attributes.Title });
                    }
                }
            }
        }

        return View("Detail", model);
    }

And my Model:

public class ProductListWidgetModel
{
    /// <summary>
    /// Gets or sets all the messages.
    /// </summary>
    public string Message { get; set; }
    public string Title { get; set; }
    public string SubTitle { get; set; }

    public Product Product { get; set; }
    public List<Product> ProductsList { get; set; }

    public List<ProductVariation> ProductVariations { get; set; }

    public List<SelectListItem> Portions { get; set; }
    public List<SelectListItem> Meals { get; set; }
}

And also my View:

<table>
    <tr>
        <th id="row">@Html.DropDownList("MealsSelector", @Model.Meals } )</th>
        <td id="row">Middager</td>
    </tr>
    <tr>
        <th id="row">@Html.DropDownList("PortionsSelector", @Model.Portions)</th>
        <td id="row">Porsjoner</td>
    </tr>
</table>

But what I really need to do is have the Portions bound DDL only show the possible values based on the Days DDL, so if I selected 5 Days I should only have the choice of selecting 4 Portions on the Portions DDL.

How can I achieve this? Ideally I would prefer to have this done via Ajax or a simple LINQ filter to do this.

Can anyone guide me on how to bind these DDLs to one another?