问RavenDb -具有过滤group by结果的多映射索引

EN

public class Race {
    public string Id { get; set; }
    public string Name { get; set; }
    public string Distance { get; set; }
    public DateTime DateOfEvent { get; set; }
    public string Location { get; set; }

}
    public class Runner {
    public string Id { get; set; }
    public string Name { get; set; }
    public DateTime DateOfBirth { get; set; }
    public string Hometown { get; set; }

    public List<string> Friends { get; set; }
    public List<string> RacesRun { get; set; }

}

public class RacesByFriends : AbstractMultiMapIndexCreationTask<RacesByFriends.ReduceResult> {

    public class ReduceResult {
        public string RunnerId { get; set; }
        public string RaceId { get; set; }
        public int Count { get; set; }
    }

    public RacesByFriends() {

        AddMap<Runner>(runners => from runner in runners
                                  from race in runner.RacesRun
                                  select new {
                                      RunnerId = runner.Id,
                                      RaceId = race,
                                      Count = 1,
                                  });

        AddMap<Race>(races => from race in races
                              select new {
                                  RunnerId = (string)null,
                                  RaceId = race.Id,
                                  Count = 0,
                              });

        Reduce = results => from result in results
                            group result by new {
                                result.RunnerId,
                                result.RaceId
                            } into g
                            select new {
                                RunnerId = g.Select(x => x.RunnerId).FirstOrDefault(),
                                RaceId = g.Select(x => x.RaceId).FirstOrDefault(),
                                Count = g.Sum(x => x.Count)
                            };

    }
}