Get average of last n-values

gistfile1.cs

using System;
using System.Linq;
using Raven.Client.Embedded;
using Raven.Client.Indexes;
using Raven.Client.Linq;

namespace ConsoleApplication
{
    class Program
    {
        class PlayerPins
        {
            public string Player { get; set; }
            public DateTime Date { get; set; }
            public int Pins { get; set; }
        }

        class Pins_ByMatchDate : AbstractIndexCreationTask<PlayerPins, Pins_ByMatchDate.Result>
        {
            public Pins_ByMatchDate()
            {
                Map = pins => from pin in pins
                              select new
                                 {
                                     pin.Player,
                                     pin.Date,
                                     pin.Pins
                                 };

                Reduce = maps => from map in maps
                                 group map by map.Player into grouped
                                 select new
                                 {
                                     Player = grouped.Key, 
                                     grouped.OrderByDescending(x => x.Date).Take(3).First().Date,
                                     Pins = grouped.OrderByDescending(x => x.Date).Take(3).Average(x => x.Pins),
                                 };
            }

            public class Result
            {
                public string Player { get; set; }
                public DateTime Date { get; set; }
                public double Pins { get; set; }
            }
        }

        static void Main()
        {
            var playerPins = new[]
                                 {
                                     new PlayerPins {Player = "John", Date = new DateTime(2011, 10, 11), Pins = 200},
                                     new PlayerPins {Player = "John", Date = new DateTime(2011, 11, 11), Pins = 250},
                                     new PlayerPins {Player = "John", Date = new DateTime(2011, 11, 12), Pins = 240},
                                     new PlayerPins {Player = "John", Date = new DateTime(2011, 11, 13), Pins = 260},
                                     new PlayerPins {Player = "John", Date = new DateTime(2011, 11, 14), Pins = 270},
                                     new PlayerPins {Player = "Pete", Date = new DateTime(2010, 11, 11), Pins = 300},
                                     new PlayerPins {Player = "Pete", Date = new DateTime(2011, 11, 11), Pins = 300},
                                     new PlayerPins {Player = "Pete", Date = new DateTime(2011, 11, 12), Pins = 300},
                                     new PlayerPins {Player = "Pete", Date = new DateTime(2011, 11, 13), Pins = 300},
                                     new PlayerPins {Player = "Kara", Date = new DateTime(2011, 10, 11), Pins = 100},
                                     new PlayerPins {Player = "Kara", Date = new DateTime(2011, 10, 12), Pins = 200},
                                     new PlayerPins {Player = "Kara", Date = new DateTime(2011, 10, 13), Pins = 300}
                                 };


            using (var store = new EmbeddableDocumentStore { RunInMemory = true })
            {
                store.Initialize();

                IndexCreation.CreateIndexes(typeof(Pins_ByMatchDate).Assembly, store);

                using (var session = store.OpenSession())
                {
                    foreach(var playerPin in playerPins)
                    {
                        session.Store(playerPin);
                    }

                    session.SaveChanges();
                }

                using (var session = store.OpenSession())
                {
                    var lastAveragePins = session.Query<Pins_ByMatchDate.Result, Pins_ByMatchDate>()
                        .AsProjection<Pins_ByMatchDate.Result>()
                        .Customize(x => x.WaitForNonStaleResults());

                    foreach (var pins in lastAveragePins)
                    {
                        Console.WriteLine("{0} {1:yyyy/MM/dd} {2}", pins.Player, pins.Date, pins.Pins);
                    }

                    // expect to see
                    // John 2011-11-14 256.666666666667
                    // Pete 2011-11-13 300
                    // Kara 2011-10-13 200
                }
            }
        }
    }
}