SelectedKeywordsExample.cs

using System.Collections.Generic;
using System.Linq;
using Sitecore.ContentSearch;
using Sitecore.ContentSearch.ComputedFields;
using Sitecore.ContentSearch.Utilities;
using Sitecore.Data;
using Sitecore.Data.Items;

namespace Examples.ComputedIndexFields
{
  public class SelectedKeywords : IComputedIndexField
  {
    public object ComputeFieldValue(IIndexable indexable)
    {
      if (indexable == null)
        return null;

      var indexableItem = indexable as SitecoreIndexableItem;

      if (indexableItem == null)
        return null;

      IEnumerable<Item> selectedKeywords = GetKeywordsOfSelfAndAncestors(indexableItem);
      return selectedKeywords.Select(x => IdHelper.NormalizeGuid(x.ID));
    }

    private static IEnumerable<Item> GetKeywordsOfSelfAndAncestors(SitecoreIndexableItem indexableItem)
    {
      List<Item> selectedKeywords = new List<Item>();
      Item currentItem = indexableItem.Item;
      ID keywordFieldId = new ID("ID-TO-KEYWORD-FIELD");

      // Here we go up the tree, visiting each ancestor node, and if it
      // contains the keyword-selector field, we extract the selected
      // keywords selected and accumulate them to the overall list of
      // keywords for the given item
      do
      {
        if (currentItem[keywordFieldId] != null)
          selectedKeywords.AddRange(new MultilistField(item.Fields[keywordFieldId]).GetItems());

        currentItem = currentItem.Parent;
      } while (currentItem != null);

      return selectedKeywords;
    }

    public string FieldName { get; set; }
    public string ReturnType { get; set; }
  }
}