QueryTokenReplacer.cs

using System;
using System.Text.RegularExpressions;
using Sitecore.Data.Items;
using Sitecore.Diagnostics;
using Sitecore.Pipelines.ExpandInitialFieldValue;

namespace CustomExpandTokenProcessors.Pipelines
{
  public class QueryTokenReplacer : ExpandInitialFieldValueProcessor
  {
    private const string Token = "$query";
    private const string Pattern = @"(\$query\((.*[^\|])\|(\w*)\))";
    private const string ItemId = "id";
    private const string ItemName = "name";

    public override void Process(ExpandInitialFieldValueArgs args)
    {
      Assert.ArgumentNotNull(args, "args");

      if (args.SourceField.Value.Length == 0 || args.Result.Contains(Token) == false)
        return;

      string fieldValue = args.Result;

      var regex = new Regex(Pattern, RegexOptions.IgnoreCase);
      Match match = regex.Match(fieldValue);

      if (match.Success)
      {
        string token = match.Groups[1].Value;
        string query = match.Groups[2].Value;
        string fieldName = match.Groups[3].Value;

        if (query.Length <= 0 || fieldName.Length <= 0)
          return;

        Item resultItem = GetItemFromQuery(query, args.TargetItem);
        if (resultItem != null)
          ReplaceTokenValue(args, token, fieldName, resultItem);
      }
      else
        Log.Error("Invalid $query() token. Expected format: $query(<query>|<fieldName>)", this);
    }

    private static void ReplaceTokenValue(ExpandInitialFieldValueArgs args, string token, string fieldName, Item resultItem)
    {
      Assert.ArgumentNotNull(args, "args");
      Assert.IsNotNullOrEmpty(token, "token");
      Assert.IsNotNullOrEmpty(fieldName, "fieldName");
      Assert.ArgumentNotNull(resultItem, "resultItem");

      switch (fieldName.ToLowerInvariant())
      {
        case ItemId:
          args.Result = args.Result.Replace(token, resultItem.ID.ToString());
          break;

        case ItemName:
          args.Result = args.Result.Replace(token, resultItem.Name);
          break;

        default:
          args.Result = string.IsNullOrEmpty(resultItem[fieldName])
            ? args.Result.Replace(token, string.Empty)
            : args.Result.Replace(token, resultItem[fieldName]);
          break;
      }
    }

    private Item GetItemFromQuery(string query, Item item)
    {
      Assert.ArgumentNotNull(item, "item");

      Item queryResultItem = null;
      try
      {
        queryResultItem = item.Axes.SelectSingleItem(query);        
      }
      catch
      {
        Log.Error(string.Format("Failed to execute query [{0}]", query) , this);        
      }
      return queryResultItem;
    }
  }
}