So I already have a custom Data Type (Training Category) utilizing a drop-down list. Every training event has this data type set to a specific category.

I also have the drop down list in the search set to pull the pre-values utilizing:

XPathNodeIterator iterator = umbraco.library.GetPreValues(1420);
iterator.MoveNext();
XPathNodeIterator preValues = iterator.Current.SelectChildren("preValue", "");

And render the values in the drop down as such:

<select>
<option>- Select -</option>
    @while (preValues.MoveNext())
    {
        string preValue = preValues.Current.Value;
        <option value="@preValue">@preValue</option>
    }
</select>

So, what I want to do is return the training events depending upon which category is selected.

Sample UI can be seen at http://updated.ramquest.com/services/training

I also include a "search by keyword" function, which searches a pre-defined list of values and works fine, so there's no issues with that piece. The macro in it's entirety is as follows:

@using System.Web;
@using System.Xml.XPath
@using umbraco.MacroEngines
@inherits DynamicNodeContext
@{string searchKeywords = string.Empty;
    if (Request["keywords"] != null)
    {
        searchKeywords = Request["keywords"].ToLower();
    }

    Dictionary<string, object> values = new Dictionary<string, object>();
    values.Add("searchString", searchKeywords);

    var items = Model.Trainings.Where("trainingTags.ToLower().Contains(searchString)", values).OrderBy("startDate");

    XPathNodeIterator iterator = umbraco.library.GetPreValues(1420);
    iterator.MoveNext();
    XPathNodeIterator preValues = iterator.Current.SelectChildren("preValue", "");


    <form name="searchForm" method="post">
        <div class="keywordsSearchBox">
            <h4>Search Training</h4>
            <div class="searchSection">
                <label>Search by Keyword: </label>
                <input type="text" id="keywords" name="keywords" value="@searchKeywords" />
                <p class="searchSeparator">-or-</p>
                    <label>Search by Product: </label>
                    <select>
                        <option>- Select -</option>
                        @while (preValues.MoveNext())
                        {
                            string preValue = preValues.Current.Value;
                            <option value="@preValue">@preValue</option>
                        }
                    </select>
                <input type="submit" value="Search" />
            </div>
        </div>
    </form>

    <div class="searchResults">
    <p><strong>Below are the next scheduled RamQuest trainings. Search for specific topics or keywords, or search "all" to see a complete list of RamQuest's training topics and dates.</strong></p>

    @if (items.Count() > 0 && searchKeywords == "")
        {
            <ul class="searchResultList">
                @foreach (var item in items.Take(3))
                {

                    <li>
                        <h3 class="headline">
                            <a href="@item.Url">@item.Title</a>
                        </h3>
                        <span>
                            <b>When: </b> @umbraco.library.ShortDate(@item.startDate.ToString(), false, "")
                            <br />
                            <b>Time: </b>@item.time
                            <br />
                            <b>Cost: </b> @item.cost
                            <br />
                            <b>Location: </b> @item.trainingLocation
                        </span>
                    </li>
                }
            </ul>
        }
    else if (items.Count() > 0 && searchKeywords != "")
        {
            <ul class="searchResultList">
                @foreach (var item in items)
                {

                    <li>
                        <h3 class="headline">
                            <a href="@item.Url">@item.Title</a>
                        </h3>
                        <span>
                            <b>When: </b> @umbraco.library.ShortDate(@item.startDate.ToString(), false, "")
                            <br />
                            <b>Time: </b> @item.time
                            <br />
                            <b>Cost: </b> @item.cost
                            <br />
                            <b>Location: </b> @item.location
                        </span>
                    </li>
                }
            </ul>
        }
    else if (items.Count() == 0 && Request["keywords"] != null)
        {
            <span>There are no matching training events</span>
        }
    </div>

}

So any ideas on how to call the categories when the drop box is utilized?