mongodb - Incorrect response to mapReduce query in mongo-db -

i have 1000 user records in collecton, in 459 document has gender male , remaining female

//document structure > db.user_details.find().pretty() {     "_id" : objectid("557e610d626754910f0974a4"),     "id" : 0,     "name" : "leanne flinn",     "email" : "leanne.flinn@unilogic.com",     "work" : "unilogic",     "dob" : "fri jun 11 1965 20:50:58 gmt+0530 (ist)",     "age" : 5,     "gender" : "female",     "salary" : 35696,     "hobbies" : "acrobatics,meditation,music" } {     "_id" : objectid("557e610d626754910f0974a5"),     "id" : 1,     "name" : "edward young",     "email" : "edward.young@solexis.com",     "work" : "solexis",     "dob" : "wed feb 12 1941 16:45:53 gmt+0530 (ist)",     "age" : 1,     "gender" : "female",     "salary" : 72291,     "hobbies" : "acrobatics,meditation,music" } {     "_id" : objectid("557e610d626754910f0974a6"),     "id" : 2,     "name" : "haydee milligan",     "email" : "haydee.milligan@dalserve.com",     "work" : "dalserve",     "dob" : "tue sep 13 1994 13:45:04 gmt+0530 (ist)",     "age" : 17,     "gender" : "male",     "salary" : 20026,     "hobbies" : "papier-mache" } {     "_id" : objectid("557e610d626754910f0974a7"),     "id" : 3,     "name" : "lyle keesee",     "email" : "lyle.keesee@terrasys.com",     "work" : "terrasys",     "dob" : "tue apr 25 1922 13:39:46 gmt+0530 (ist)",     "age" : 79,     "gender" : "female",     "salary" : 48032,     "hobbies" : "acrobatics,meditation,music" } {     "_id" : objectid("557e610d626754910f0974a8"),     "id" : 4,     "name" : "shea mercer",     "email" : "shea.mercer@pancast.com",     "work" : "pancast",     "dob" : "mon apr 08 1935 06:10:30 gmt+0530 (ist)",     "age" : 51,     "gender" : "male",     "salary" : 31511,     "hobbies" : "acrobatics,photography,papier-mache" } 

number of users in each gender

> db.user_details.find({gender:'male'}).count() 459 >  > db.user_details.find({gender:'female'}).count() 541    > db.user_details.find({name:{$ne:null}}).count() 1000 > db.user_details.find({age:{$ne:null}}).count() 1000 

map reduce code

mapper = function(){   emit(this.gender, {name:this.name,age:this.age}) }  reducer = function(gender, users){   var res = 0;   users.foreach(function(user){     res = res + 1   })   return res; }   db.user_details.mapreduce(mapper, reducer, {out: {inline:1}}) 

why map reduce result has 112 documents? should contain 459 , 541 male , female respectively, isn't it?

// map reduce result {   "results" : [     {       "_id" : "female",       "value" : 56     },     {       "_id" : "male",       "value" : 46     }   ],   "timemillis" : 45,   "counts" : {     "input" : 1000,     "emit" : 1000,     "reduce" : 20,     "output" : 2   },   "ok" : 1 } 

note : know not proper way use map reduce, faced more creepy problem in map reduce. once solution question solve

your problem here have missed 1 of core concepts of how mapreduce works. relevant documentation explains found here:

• mongodb can invoke reduce function more once same key. in case, previous output reduce function key become 1 of input values next reduce function invocation key.

and bit later:

• the type of return object must identical type of value emitted map function

what 2 statements mean need use exact same signature issued both mapper , reducer functions reduce process indeed called "multiple times".

this how mapreduce deals large data, not processing of same values given "key" @ once, doing in incremental "chunks":

there fore if want in output "number" "emit" "number" well:

db.collection.mapreduce(     function() {        emit(this.gender, this.age);     },     function(key,values) {         return array.sum( values )     },     { "out": { "inline": 1 } } ) 

or "count" per type:

db.collection.mapreduce(     function() {        emit(this.gender, 1);     },     function(key,values) {         return array.sum( values )     },     { "out": { "inline": 1 } } ) 

the point "you need put out same put in", "go in again". whatever data want collect, output structure both mapper , reducer must same.