db.people.aggregate([
{
    "$group": {
     "_id": "$name.lastName",
     "count": {
       "$sum": 1
     }
    }
},
{
       "$project":
          {
            "_id": 1,
            "count": 1,
            "letter": { "$substr": [ "$_id", 0, 1 ] }
          }
      },
      {"$out":"lastNames"}
], {"allowDiskUse":true})