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Map/Reduce over sharded data with RavenDB
errors/1/6
{
  "UniqueId": "c62c7e30-8ec7-45af-88e4-da023d796727",
  "ApplicationName": "MyAppName"
}

errors/2/6  --Error stored on shard node 2
{
  "UniqueId": "7e0b0f87-9d75-4e70-9fa0-d64a18bc88dc",
  "ApplicationName": "MyAppName"
}

public class ApplicationNames : AbstractIndexCreationTask<ErrorDocument, Application>
{
    public ApplicationNames()
    {
         Map = errors => from error in errors
                        select new { error.ApplicationName, Count = 1 };

        Reduce = results => from error in results
                            group error by new { error.ApplicationName, error.Count } into g
                            select new { g.Key.ApplicationName, Count = g.Sum(x=> x.Count) };
    }
}

session.Query<Application, ApplicationNames>()
   .ToList()
   .Select(new ApplicationNames().Reduce)
   .ToList();