mongodb map reduce value undefined - json

Question

I have following collection in mongodb-
> db.games.find().pretty();
{
"_id" : ObjectId("5db06c02e08772b58596ec72"),
"name" : "Cricket",
"genre" : "sport",
"rating" : 10
}
{
"_id" : ObjectId("5db06c02e08772b58596ec73"),
"name" : "Football",
"genre" : "sport",
"rating" : 100,
"achievement" : "champion",
"games" : [
{
"score" : 20
},
{
"score" : 30
},
{
"score" : 22
},
{
"score" : 145
}
]
}
{
"_id" : ObjectId("5db06c02e08772b58596ec74"),
"name" : "Ludo",
"genre" : "indoor",
"rating" : 1
}
{
"_id" : ObjectId("5db06c02e08772b58596ec75"),
"name" : "Badminton",
"genre" : "indoor",
"rating" : 60,
"games" : [
{
"score" : 34
},
{
"score" : 12
},
{
"score" : 50
}
]
}
{
"_id" : ObjectId("5db06c02e08772b58596ec76"),
"name" : "Swimming",
"genre" : "water",
"rating" : 50
}
{
"_id" : ObjectId("5db06c02e08772b58596ec77"),
"name" : "Running",
"genre" : "atheletics",
"rating" : 70
}
{
"_id" : ObjectId("5db06c02e08772b58596ec78"),
"name" : "Shotput",
"genre" : "atheletics",
"rating" : 66
}
I want to sum of scores for each game in the collection. For this I am trying to implement map reduce as follows -
> db.games.mapReduce( function(){emit(this.name,this.score)}, function(key,values) {return Array.sum(values)}, {out:"out_scores"});
{
"result" : "out_scores",
"timeMillis" : 330,
"counts" : {
"input" : 7,
"emit" : 7,
"reduce" : 0,
"output" : 7
},
"ok" : 1
}
But the values in the resulting collection as undefined as follows -
db.out_scores.find().pretty();
{ "_id" : "Badminton", "value" : undefined }
{ "_id" : "Cricket", "value" : undefined }
{ "_id" : "Football", "value" : undefined }
{ "_id" : "Ludo", "value" : undefined }
{ "_id" : "Running", "value" : undefined }
{ "_id" : "Shotput", "value" : undefined }
{ "_id" : "Swimming", "value" : undefined }
I am expecting sum of values of scores for each game. What am I doing wrong here?

Answer 1

Perhaps this query might help?
db.games.aggregate(
[
{ $unwind: { path: "$games", preserveNullAndEmptyArrays: true }},
{ $group: {
_id: "$name",
score: { $sum: "$games.score" }
}}
])
Results:
{ "_id" : "Shotput", "score" : 0 }
{ "_id" : "Running", "score" : 0 }
{ "_id" : "Ludo", "score" : 0 }
{ "_id" : "Swimming", "score" : 0 }
{ "_id" : "Badminton", "score" : 96 }
{ "_id" : "Football", "score" : 217 }
{ "_id" : "Cricket", "score" : 0 }
** If still want to use mapreduce...**
The root problem is nested data. Need to refer to the data from the games.score perspective. The mapping function of map reduce requires this logic.
db.games.mapReduce( function(){
var sum_of_score = 0;
if (this.games != undefined) {
for (var i=0; i<this.games.length; i++) {
sum_of_score += this.games[i].score;
}
}
emit(this.name, sum_of_score)
}, function(key, values) { }, {out:"out_scores"});
Query:
db.out_scores.find()
Results:
{ "_id" : "Badminton", "value" : 96 }
{ "_id" : "Cricket", "value" : 0 }
{ "_id" : "Football", "value" : 217 }
{ "_id" : "Ludo", "value" : 0 }
{ "_id" : "Running", "value" : 0 }
{ "_id" : "Shotput", "value" : 0 }
{ "_id" : "Swimming", "value" : 0 }