mongodb: reduce function not action when only one result is there - mongodb

Question

I need to do the weighted average.
Did the coding as below
db.runCommand(
{ mapreduce : "<collecton>" ,
map: function ()
{
emit ({nkey: this.nkey}, {price: this.tags["31"], qty: this.tags["32"]});
},
reduce: function(key, vals)
{
var ret = {wavg:0};
var mul1 = 0.0;
var sum1 = 0.0;
for (var i=0;i<vals.length;i++)
{ mul1 += vals[i].price * vals[i].qty;
sum1 += vals[i].qty;
}
ret.wavg = mul1/sum1;
return ret;
},
out: 'res2', verbose: true
}
);
> db.res2.find()
{ "_id" : { "nkey" : "key1" }, "value" : { "wavg" : 311.7647058823529 } }
{ "_id" : { "nkey" : "ke2" }, "value" : { "wavg" : 585.7142857142857 } }
{ "_id" : { "nkey" : "key3" }, "value" : { "price" : 1000, "qty" : 110 } }
{ "_id" : { "nkey" : "key4" }, "value" : { "wavg" : 825 } }
If you notice, in the final reducer output(third row), it dint actually go thru the reduce functionality. The key occur only once, hence one result will be emitted. But I still want the reduce function to be acting on that to get the weighted average. I can't just go ahead with price and qty wherever I have only one occurence of the key, where I need weighted average for that key also.
Is there any way to achieve this ?

Answer 1

This is essentially how mapReduce works in that the reducer is never called when you only have one result. But you can always alter such results with a finalize stage:
db.runCommand({
"mapreduce" : "<collecton>" ,
"map": function () {
emit (
{ "nkey": this.nkey},
{ "price": this.tags["31"], qty: this.tags["32"]}
);
},
"reduce": function(key, vals) {
var ret = { "wavg": 0 };
var mul1 = 0.0;
var sum1 = 0.0;
for ( var i=0; i<vals.length; i++ ) {
mul1 += vals[i].price * vals[i].qty;
sum1 += vals[i].qty;
}
ret.wavg = mul1/sum1;
return ret;
},
"finalize": function(key,value) {
if (value.hasOwnProperty("price") {
value.wavg = value.price;
delete value["price"];
}
return value;
},
"out": 'res2',
"verbose": true
});
Or otherwise alternately just sum your keys in the reduce stage and do all the division in the finalize stage if that suits you thinking better. But then you would need to do your "multiplication" part in the "mapper" for that to work.