string template example - lookup pipeline

Program.cs

using MongoDB.Bson;
using MongoDB.Bson.Serialization.Attributes;
using MongoDB.Driver;
using MongoDB.Entities;
using MongoDB.Entities.Core;
using System.Linq;

namespace StackOverFlow
{
    public class Channel : Entity
    {
        public string Name { get; set; }
        public Episode[] Episodes { get; set; }
    }

    public class Episode
    {
        public string Name { get; set; }

        [BsonRepresentation(BsonType.ObjectId)]
        public string[] Tracks { get; set; }
    }

    public class Track : Entity
    {
        public string Language { get; set; }
    }

    public class EpisodeWithTracks
    {
        public string Name { get; set; }
        public Track[] Tracks { get; set; }
    }

    public static class Program
    {
        private static void Main()
        {
            new DB("test");

            var trk1 = new Track { Language = "english" };
            var trk2 = new Track { Language = "spanish" };
            new[] { trk1, trk2 }.Save();

            new Channel
            {
                Name = "channel one",
                Episodes = new[] {
                    new Episode{
                        Name = "first episode",
                        Tracks = new[]{ trk1.ID, trk2.ID} }
                }
            }.Save();

            var pipeline = new Template<Channel, EpisodeWithTracks>(@"
            [
                {
                    '$unwind': '$<Episodes>'
                },
                {
                    '$lookup': {
                        'from': '<track_collection>',
                        'localField': '<Episodes.Tracks>',
                        'foreignField': '_id',
                        'as': '<Tracks>'
                    }
                }
            ]")

            .Path(c => c.Episodes)
            .Tag("track_collection", trk1.CollectionName())
            .Path(c => c.Episodes[0].Tracks)
            .PathOfResult(ewt => ewt.Tracks);

            var result = DB.Aggregate(pipeline)
                            .ToList();
        }
    }
}