Using AbstractMultiMapIndexCreationTask to replace left joins in SQL

BabySittingTransaction.cs

public class BabySittingTransaction : DomainBase
    {
        public virtual int ChildrenWatched { get; set; }

        public virtual int Hours { get; set; }

        public virtual string SittingProvider { get; set; }

        public virtual string SittingReceiver { get; set; }

        public virtual DateTimeOffset StartedAtUtc { get; set; }
    }

SitterProviderScoreIndexTask.cs

public class SitterProviderScoreIndexTask : AbstractMultiMapIndexCreationTask<SittingProviderScore>
    {
        public SitterProviderScoreIndexTask()
        {
            AddMap<BabySittingTransaction>(
                providers => from sitting in providers
                             select new
                             {
                                 Sitter = sitting.SittingProvider,
                                 Points = sitting.ChildrenWatched * sitting.Hours
                             }
                );

            AddMap<BabySittingTransaction>(
                 receivers => from sitting in receivers
                              select new
                              {
                                  Sitter = sitting.SittingReceiver,
                                  Points = -1 * sitting.ChildrenWatched * sitting.Hours
                              }
                );


            Reduce = results => from score in results
                                group score by score.Sitter
                                into g
                                select new { Sitter = g.Key, Points = g.Sum(x => x.Points) };
            
            
            
            Sort(x => x.Points, SortOptions.Int);
        }
    }