Lucene index category

Computed Index Field

namespace MyProject.ComputedIndexFields
{
    /// <summary>
    /// Index the main level section the item falls under
    /// </summary>
    public class IndexCategory : IComputedIndexField
    {
        /// <inheritdoc />
        public string FieldName { get; set; }
        /// <inheritdoc />
        public string ReturnType { get; set; }

        /// <inheritdoc />
        public object ComputeFieldValue(IIndexable indexable)
        {
            Item item = indexable as SitecoreIndexableItem;

            if (item != null && !item.Paths.IsMediaItem)
            {
                try
                {
                    if (item.ParentID == SearchConstants.rootItem)
                    {
                        if (item.TemplateID == SearchConstants.landingTemplate)
                        {
                            // landing page item - return itself
                            return item.DisplayName;
                        }
                        else
                        {
                            // Root level item which is not landing page
                            return null;
                        }
                    }

                    return RecursiveParent(item);
                }
                catch (Exception)
                {
                    return null;
                }
            }
           
            return null; // Return null if nothing to index
        }

        private static string RecursiveParent(Item currentItem)
        {
            if (currentItem.ParentID == SearchConstants.sitecoreItem)
            {
                // went too far
                return null;
            }

            if (currentItem.ParentID == SearchConstants.rootItem)
            {
                // parent item is the root, return current item
                return currentItem.DisplayName;
            }

            return RecursiveParent(currentItem.Parent);
        }
    }
}

Search constants

public static class SearchConstants
{
  public static ID rootItem = new ID("{A7CDAD31-BE99-4E67-A86C-2D1792F5564A}"); // Home item
  public static ID sitecoreItem = new ID("{11111111-1111-1111-1111-111111111111}"); // Top level item in tree (Sitecore)
  public static ID landingTemplate = new ID("{A81FB275-022F-430C-8C02-A4B526F8981A}"); // landing page template ID
}

Search index XML

<fieldMap type="Sitecore.ContentSearch.FieldMap, Sitecore.ContentSearch">
  <fieldNames hint="raw:AddFieldByFieldName">
    !-- Category (root parent item) for faceting-->
    <field fieldName="categoryfacet" storageType="YES" indexType="TOKENIZED" vectorType="NO" boost="1f" type="System.String" settingType="Sitecore.ContentSearch.LuceneProvider.LuceneSearchFieldConfiguration, Sitecore.ContentSearch.LuceneProvider">
      <Analyzer type="Sitecore.ContentSearch.LuceneProvider.Analyzers.LowerCaseKeywordAnalyzer, Sitecore.ContentSearch.LuceneProvider" />
    </field>
  </fieldNames>
</fieldMap>

<fields hint="raw:AddComputedIndexField">
  <!-- Category (root parent item) for faceting -->
  <field fieldName="categoryfacet" storageType="yes" indexType="untokenized" 
    patch:after="field[last()]">WCC.Internet.UI.ComputedIndexFields.IndexCategory, WCC.Internet.UI</field>
  </fields>