Example for SO Q14228793

Q14228793.cs

using System.Collections.Generic;
using System.Linq;
using Raven.Client;
using Raven.Client.Indexes;
using Raven.Client.Linq;
using Raven.Tests.Helpers;
using Xunit;

namespace RavenTests.StackOverflow
{
    public class Q14228793 : RavenTestBase
    {
        [Fact]
        public void Test()
        {
            using (var documentStore = NewDocumentStore())
            {
                documentStore.ExecuteIndex(new Photos_ByTag());

                using (var session = documentStore.OpenSession())
                {
                    session.Store(new Album { Id = "albums/1", Name = "Album 1" });
                    session.Store(new Photo { Id = "photos/1", AlbumId = "albums/1", Tags = new[] { "alice", "bob", "charlie" } });
                    session.Store(new Photo { Id = "photos/2", AlbumId = "albums/1", Tags = new[] { "alice", "david" } });
                    session.Store(new Photo { Id = "photos/3", AlbumId = "albums/1", Tags = new[] { "bob", "david" } });

                    session.Store(new Album { Id = "albums/2", Name = "Album 2" });
                    session.Store(new Photo { Id = "photos/4", AlbumId = "albums/2", Tags = new[] { "bob", "charlie" } });
                    session.Store(new Photo { Id = "photos/5", AlbumId = "albums/2", Tags = new[] { "charlie", "david" } });
                    session.Store(new Photo { Id = "photos/6", AlbumId = "albums/2", Tags = new[] { "bob", "alice" } });

                    session.SaveChanges();
                }

                using (var session = documentStore.OpenSession())
                {
                    var tagsToSearch = new[] { "charlie", "bob" };

                    var photos = session.Query<Photos_ByTag.IndexEntry, Photos_ByTag>()
                                        .Customize(x => x.WaitForNonStaleResults()) // only in testing
                                        .Where(x => x.Tag.In(tagsToSearch))
                                        .As<Photo>()
                                        .ToList();

                    Assert.Equal(5, photos.Count);
                    Assert.DoesNotContain("photos/5", photos.Select(x => x.Id));
                }
            }
        }

        public class Album
        {
            public string Id { get; set; }
            public string Name { get; set; }
        }

        public class Photo
        {
            public string Id { get; set; }
            public string AlbumId { get; set; }
            public IList<string> Tags { get; set; }
        }

        public class Photos_ByTag : AbstractIndexCreationTask<Photo, Photos_ByTag.IndexEntry>
        {
            public class IndexEntry
            {
                public string Tag { get; set; }
            }

            public Photos_ByTag()
            {
                Map = photos => from photo in photos
                                from tag in photo.Tags
                                select new {
                                               Tag = tag
                                           };
            }
        }
    }
}