Problems with mapreduce function to generate counts MongoDB JavaScript

Working with Data
1

Hi, I’m trying to calculate the number of trips (trip_I) that each route (route_I) has with MapReduce function, but doesn’t work. They also ask me to use a finalize function but I have not been able to figure out how. This is the data:

{    "_id" : ObjectId("602018db89877fd06f61d2f2"),    "from_stop_I" : 1908,    "to_stop_I" : 1491,    "dep_time" : ISODate("2016-09-05T07:40:00.000Z"),    "arr_time" : ISODate("2016-09-05T07:41:00.000Z"),     "route_type" : 3,    "trip_I" : 18,    "seq" : 37,    "route_I" : 104} 
{    "_id" : ObjectId("602018db89877fd06f61d2f3"),    "from_stop_I" : 1491,    "to_stop_I" : 1500,    "dep_time" : ISODate("2016-09-05T07:41:00.000Z"),    "arr_time" : ISODate("2016-09-05T07:44:00.000Z"),     "route_type" : 3,    "trip_I" : 18,    "seq" : 38,    "route_I" : 104}
{    "_id" : ObjectId("602018dc89877fd06f61d721"),    "from_stop_I" : 1156,    "to_stop_I" : 1158,    "dep_time" : ISODate("2016-09-05T08:06:00.000Z"),    "arr_time" : ISODate("2016-09-05T08:06:00.000Z"),    "route_type" : 3,    "trip_I" : 72,    "seq" : 1,    "route_I" : 104}
{    "_id" : ObjectId("602018dc89877fd06f61d722"),    "from_stop_I" : 1158,    "to_stop_I" : 1160,    "dep_time" : ISODate("2016-09-05T08:06:00.000Z"),    "arr_time" : ISODate("2016-09-05T08:07:00.000Z"),    "route_type" : 3,    "trip_I" : 72,    "seq" : 2,    "route_I" : 104}
{    "_id" : ObjectId("602018dc89877fd06f61d746"),    "from_stop_I" : 1491,    "to_stop_I" : 1500,    "dep_time" : ISODate("2016-09-05T08:27:00.000Z"),    "arr_time" : ISODate("2016-09-05T08:30:00.000Z"),    "route_type" : 3,    "trip_I" : 72,    "seq" : 38,    "route_I" : 104}
{    "_id" : ObjectId("6020193c89877fd06f639dec"),    "from_stop_I" : 1156,    "to_stop_I" : 1158,    "dep_time" : ISODate("2016-09-05T23:20:00.000Z"),    "arr_time" : ISODate("2016-09-05T23:20:00.000Z"),    "route_type" : 3,    "trip_I" : 6972,    "seq" : 1,    "route_I" : 104}
{    "_id" : ObjectId("6020193c89877fd06f639ded"),    "from_stop_I" : 1158,    "to_stop_I" : 1160,    "dep_time" : ISODate("2016-09-05T23:20:00.000Z"),    "arr_time" : ISODate("2016-09-05T23:21:00.000Z"),    "route_type" : 3,    "trip_I" : 6972,    "seq" : 2,    "route_I" : 104}
{    "_id" : ObjectId("6020193c89877fd06f639dee"),    "from_stop_I" : 1160,    "to_stop_I" : 1162,    "dep_time" : ISODate("2016-09-05T23:21:00.000Z"),    "arr_time" : ISODate("2016-09-05T23:21:00.000Z"),    "route_type" : 3,    "trip_I" : 6972,    "seq" : 3,    "route_I" : 104}

This is the code:

function mapFunction () {
    var key = this.route_I;
    var value = { totalTrips: this.trip_I,totalRegistros:1 };
    emit( key, value );
};

function reduceFunction (key, trips) {
   var reducedObject = { totalTrips: 0,totalRegistros:0 };
   trips.forEach(function(value) {
      reducedObject.totalTrips ++;
      reducedObject.totalRegistros += value.totalRegistros;
   })
   ;
   return reducedObject;
};

function finalizeFunction(key, reducedObject) {   
      
//???
   return reducedObject;
};

db.routes.mapReduce(
   mapFunction,
   reduceFunction,
   {
     out: "routes_out",
    finalize: finalizeFunction     
   }
)

The output should be 3 trips for route 104. Thanks

2

Hi @Johanna_Valenzuela_S,

Welcome to MongoDB community.

Why not to use an aggregation $group stage and count the trips?

https://docs.mongodb.com/manual/reference/operator/aggregation/sum/#use-in--group-stage

This is the recommended way.

Thanks
Pavel

5

Hi @Pavel_Duchovny , the professor asks me to use mapreduce only. With $group I know how do that, but it has to be with mapreduce :frowning_face:

6

Hi @Johanna_Valenzuela_S,

I see. Well there are some examples on the map reduce command including a sum per customer.

This sounds exactly what you need suming 1 per document:
https://docs.mongodb.com/manual/reference/command/mapReduce/#return-the-total-price-per-customer

Thanks
Pavel