when I use the map-reduce of mongodb, why does it nest?

Question

I insert into mongo with the code:

for (var i=0; i<1000; i++) {
db.test.insert({ "name" : "user"+i, "age":i%17 , "created_at" : new Date() });
} 

and the map function is

var m = function(){ emit(this.age,this.name); }

the reduce function is

var r=function(key,values){ var ret={age:key,names:values}; return ret;}

But, when I run the code

db.runCommand({mapreduce:"test",map:m,reduce:r,     out:"t_age_names"} )

I get 17 json and everyone is like

{ "_id" : 8, "value" : { "age" : 8, "names" : [ { "age" : 8, "names" : [ { "age" : 8, "names" : [ "user8", "user25", "user42", "user59", "user76", "user93", "user110", "user127", "user144", "user161", "user178", "user195" ] }, "user212", "user229", "user246", "user263", "user280", "user297", "user314", "user331", "user348", "user365", "user382", "user399" ] }, { "age" : 8, "names" : [ { "age" : 8, "names" : [ "user416", "user433", "user450", "user467", "user484", "user501", "user518", "user535", "user552", "user569", "user586" ] }, "user603", "user620", "user637", "user654", "user671", "user688", "user705", "user722", "user739", "user756", "user773", "user790" ] }, { "age" : 8, "names" : [ "user807", "user824", "user841", "user858", "user875", "user892", "user909", "user926", "user943", "user960", "user977", "user994" ] } ] } }

Why does it nest?

Why does it nest?
And the length of every array is 12, it seems like to be caused by emit function, is it?

Answer 1

Your reduce violates [at least] one of the requirements :

the type of the return object must be identical to the type of the value emitted by the map function.

So either change what reduce outputs, or what map does.