Sitecore computed facet field

gistfile1.txt

    public class RelatedThemesFacet : IComputedIndexField
    {
        public string FieldName { get; set; }
        public string ReturnType { get; set; }

        public object ComputeFieldValue(IIndexable indexable)
        {
            var indexableItem = indexable as SitecoreIndexableItem;

            if (indexableItem == null || indexableItem.Item == null)
            {
                return false;
            }
            
            return GetReferringItems(indexableItem, "Theme");
        }
        
        private List<Item> GetReferringItems(Item item)
        {
            if (_referringItems.ContainsKey(item.ID.ToString()))
            {
                return _referringItems[item.ID.ToString()];
            }

            var itemLinks = Globals.LinkDatabase.GetItemReferrers(item, false);
            var items = new List<Item>();

            itemLinks.ForEach(x => items.Add(x.GetSourceItem()));

            _referringItems[item.ID.ToString()] = items;

            return items;
        }
        
        private List<Item> GetReferringItems(Item item, string templateFilter)
        {
            return FilterListOfItemsByTemplate(GetReferringItems(item), templateFilter);
        }

        private List<Item> FilterListOfItemsByTemplate(List<Item> items, string template)
        {
            if (items == null)
            {
                return new List<Item>();
            }

            return items.Where(x => x != null && x.TemplateID.ToString() == template).ToList();
        }
    }