Load First of Many

gistfile1.cs

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

namespace Testing
{
  public class FirstOfManyTests : RavenTestBase
	{
		[Fact]
		public void ShouldFindPlace()
		{
			using (var store = NewDocumentStore())
			{
				new PlaceWithCoverPhotoIndex().Execute(store);

				using (var session = store.OpenSession())
				{
					session.Store(new Place() { Id = 1, Name = "Egypt", PhotoIds = new[] {10, 11, 12}});
					session.Store(new Photo() { Id = 10, Title = "Pyramid"});
					session.Store(new Photo() { Id = 11, Title = "Nile" });
					session.Store(new Photo() { Id = 12, Title = "Sphinx" });
					session.SaveChanges();
				}

				WaitForIndexing(store);

				using (var session = store.OpenSession())
				{
					var list = session
								.Query<Place, PlaceWithCoverPhotoIndex>()
								.ToList();

					Assert.Empty(store.DatabaseCommands.GetStatistics().Errors);
					Assert.Equal(1, list.Count);
				}
			}
		}
		// documents: Place
		public class Place
		{
			public int Id { get; set; }
			public string Name { get; set; }
			public IList<int> PhotoIds { get; set; }
		}

		// documents: Photo
		public class Photo
		{
			public int Id { get; set; }
			public string Title { get; set; }
		}

		// index definition
		public class PlaceWithCoverPhotoIndex : AbstractIndexCreationTask<Place>
		{

			public PlaceWithCoverPhotoIndex()
			{

				Map = places =>
				      from place in places
				      let firstPhotoId = place.PhotoIds.FirstOrDefault()
				      select new { CoverPhotoId = firstPhotoId };

				TransformResults =
					(database, places) =>
					from place in places
					let firstPhotoId = "photos/" + place.PhotoIds.FirstOrDefault()
					let cover = database.Load<Photo>(firstPhotoId)
					select new
					{
						Id = place.Id,
						Name = place.Name,
						CoverPhoto = new
						{
							PhotoId = cover.Id,
							Title = cover.Title,
						}
					};

			}
		}
	}
}

gistfile2.cs

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

namespace Testing
{
	public class FirstOfManyTests : RavenTestBase
	{
		[Fact]
		public void ShouldFindPlace()
		{
			using (var store = NewDocumentStore())
			{
				new PlaceWithCoverPhotoIndex().Execute(store);

				using (var session = store.OpenSession())
				{
					session.Store(new Place() { Id = "places/1", Name = "Egypt", PhotoIds = new[] { "photos/10", "photos/11", "photos/12" } });
					session.Store(new Photo() { Id = "photos/10", Title = "Pyramid" });
					session.Store(new Photo() { Id = "photos/11", Title = "Nile" });
					session.Store(new Photo() { Id = "photos/12", Title = "Sphinx" });
					session.SaveChanges();
				}

				WaitForIndexing(store);

				using (var session = store.OpenSession())
				{
					var list = session
								.Query<Place, PlaceWithCoverPhotoIndex>()
								.OfType<PlaceWithCoverPhotoIndex.Result>()
								.ToList();

					Assert.Empty(store.DatabaseCommands.GetStatistics().Errors);
					Assert.Equal(1, list.Count);
				}
			}
		}
		// documents: Place
		public class Place
		{
			public string Id { get; set; }
			public string Name { get; set; }
			public IList<string> PhotoIds { get; set; }
		}

		// documents: Photo
		public class Photo
		{
			public string Id { get; set; }
			public string Title { get; set; }
		}

		// index definition
		public class PlaceWithCoverPhotoIndex : AbstractIndexCreationTask<Place>
		{
			public class Result
			{
				public string Id { get; set; }
				public string Name { get; set; }
				public Photo CoverPhoto { get; set; }
			}

			public PlaceWithCoverPhotoIndex()
			{
				Map = places =>
				      from place in places
				      select new { place.Name };

				TransformResults =
					(database, places) =>
					from place in places
					let cover = place.PhotoIds.Select(x => database.Load<Photo>(x)).FirstOrDefault()
					select new
					{
						Id = place.Id,
						Name = place.Name,
						CoverPhoto = cover
					};
			}
		}
	}
}

gistfile3.cs

session.Store(new Place() { Id = "places/1", Name = "Egypt", PhotoIds = new[] { "places/1/photos/10", "places/1/photos/11", "places/1/photos/12" } });
session.Store(new Photo() { Id = "places/1/photos/10", Title = "Pyramid" });
session.Store(new Photo() { Id = "places/1/photos/11", Title = "Nile" });
session.Store(new Photo() { Id = "places/1/photos/12", Title = "Sphinx" });