MongoDB Map Reduce - Finalize to skip some results

Question

I have a map reduce function that works on a collection as follows:

function Map() {
    emit (
        this.name, 
        { 
            count : 1,
            flag : this.flag

        }
    );
}
function Reduce(key, values) {
    var count = 0;
    var flag = false;

    for (var i in values){
        count = count + 1;
        if (i.flag)
             flag = true;
    }

    var reduced = {
        count : count,
        flag  : flag
    }
    return reduced;
}
function Finalize(key, reduced) {

    if (reduced.count>10 || reduced.flag){
        var finalized = {
            "count" : reduced.count 
        }
        return reduced;
    }

    return null;
}

What I am trying to do is that the Finalize will only return objects that pass a certain threshold (e.g. count>10). Currently it is still returning objects and the count is just null.

Any ideas?

Derick · Accepted Answer · 2013-08-06T09:34:14.180

3

I would suggest you use the aggregation framework instead as it's much faster and easier to understand. Your above M/R/F can be written as easily as:

db.so.insert( { name: "Derick" } );
db.so.insert( { name: "Derick" } );
db.so.insert( { name: "Derick" } );
db.so.insert( { name: "Derick" } );
db.so.insert( { name: "Checklist" } );
db.so.insert( { name: "Checklist" } );
db.so.insert( { name: "Whoop" } );
db.so.insert( { name: "Whoop", flagged: true } );
db.so.aggregate( [
    { $group: { 
        _id: '$name', 
        count: { $sum: 1 }, 
        flagged: { $sum: 
            { $cond: [ { $eq: [ '$flagged', true ] }, 1, 0 ] } 
        } 
    } },
    { $match: { $or: [ { count: { $gt: 3 } }, { flagged: { $gt: 0 } } ] } }
] );

Which returns:

{
    "result" : [
        {
            "_id" : "Whoop",
            "count" : 2,
            "flagged" : 1
        },
        {
            "_id" : "Derick",
            "count" : 4,
            "flagged" : 0
        }
    ],
    "ok" : 1
}

edited Aug 06 '13 at 09:34

answered Aug 05 '13 at 15:53

Derick

35,169
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Thanks Derick. I can't use the aggregation as there is some more logic to the map/reduce which I didn't put in for simplification of the question. – checklist Aug 05 '13 at 17:44
Please provide that in the question... A/F is more powerful than you think! – Derick Aug 05 '13 at 21:42
I have added a more complex example where the count should be more than 3 OR one of the items is flagged – checklist Aug 06 '13 at 07:41
Just fixed that in the match for you... $match is just a query. – Derick Aug 06 '13 at 09:23
This doesn't solve the mapReduce finalize filter question that is required for complex ops, or at leats more complex than aggregate can handle. – jmdiego Jan 17 '17 at 18:55

score 2 · Answer 2 · answered Nov 29 '18 at 15:50

2

You can't.
Finalize will can only change the format of your value, but you cannot cancel out the key (which is actually dictated by the map phase emits).

answered Nov 29 '18 at 15:50

igorludi

1,519
2
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MongoDB Map Reduce - Finalize to skip some results

2 Answers2