A RavenDB MultiMap Index to query across documents on non-id key.

Maps

AddMap<Pixel>(pixels =>
    from p in pixels
    from tag in (p.Tags == null || !p.Tags.Any())
        ? new[] {"_notags_"}
        : p.Tags
    select new GroupedResult
        {
            Tag = tag,
            PixelIds = new[] {p.Id},
            Urls = new[] {(string) null}
        });

AddMap<UrlTagsContainer>(urlTagsContainers =>
    from u in urlTagsContainers
    from tag in u.Tags
    select new GroupedResult
      {
          Tag = tag,
          PixelIds = new string[0],
          Urls = new[] {u.Url},
      });

PixelSearchIndex.cs

public class PixelSearchIndex : AbstractMultiMapIndexCreationTask<PixelSearchIndex.GroupedResult>
{    
    public class GroupedResult
    {
        public string Tag { get; set; }
        public IEnumerable<string> PixelIds { get; set; }
        public IEnumerable<string> Urls { get; set; }
    }

    public PixelClientSearchIndex()
    {
        AddMap<Pixel>(pixels => from p in pixels
                                from tag in (p.Tags == null || !p.Tags.Any()) ? new[] {"_notags_"} : p.Tags
                                select new GroupedResult
                                    {
                                        Tag = tag,
                                        PixelIds = new[] {p.Id},
                                        Urls = new[] {(string) null}
                                    });

        AddMap<UrlTagsContainer>(urlTagsContainers => from u in urlTagsContainers
                                                      from tag in u.Tags
                                                      select new GroupedResult
                                                          {
                                                              Tag = tag,
                                                              PixelIds = new string[0],
                                                              Urls = new[] {u.Url},
                                                          });
        Reduce = results => from result in results
                            group result by result.Tag
                            into g
                            select new GroupedResult
                                {
                                    Tag = g.Key,
                                    PixelIds = g.SelectMany(x => x.PixelIds),
                                    Urls = g.SelectMany(x => x.Urls)
                                };

        TransformResults = (database, groupedResults) => from result in groupedResults
                                                         from pixelId in result.PixelIds.Distinct()
                                                         select new
                                                             {
                                                                 Pixel = database.Load<Pixel>(pixelId),
                                                                 Urls = result.Urls.Distinct()
                                                             };
    }
}

Reduce

Reduce = results =>
    from result in results
    group result by result.Tag
    into g
    select new GroupedResult
        {
            Tag = g.Key,
            PixelIds = g.SelectMany(x => x.PixelIds),
            Urls = g.SelectMany(x => x.Urls)
        };

TransformResults

TransformResults = (database, groupedResults) =>
    from result in groupedResults
    from pixelId in result.PixelIds.Distinct()
    select new
     {
         Pixel = database.Load<Pixel>(pixelId),
         Urls = result.Urls.Distinct()
     };