Map reduce trial for MongoDB

gistfile1.js

var m = function() { 
  emit(this.id, {opaqueId: this.opaque_id}); 
}

var r = function(key, values) { 
  var all = []; 
  values.forEach(function(x) { 
    all.push(x.opaqueId);
  }); 
  return {"opaqueId": all.join(", ")};
}

res = db.activities.mapReduce(m, r, { out: 'example' });
db.example.find().pretty();
# { "_id" : null,
#   "value" : {
#      "opaqueId" : "Pkkd, ckkd, Gkkd"
#   }
# }