mongodb - Join Through Map reduce -

i have 1 collection in student_id primary key:

test1:{student_id:"xxxxx"}, 

i have collection in student_id inside array of collection:

class:{"class":"i",students:["student_id":"xxxx"]} 

my problem want join these 2 tables on basis of student id,

i using map reduce , out "merge", won't work.

my mr query follows.

db.runcommand({ mapreduce: "test1",   map : function map() {     emit(this._id,this); },  reduce : function reduce(key, values) {     return values; },   out : { merge: "testmerge"  }  });   db.runcommand({ mapreduce: "class",   map : function map() {     emit(this._id,this); },  reduce : function reduce(key, values) {     return values; },   out : { merge: "testmerge"  }  }); 

but inserts 2 rows.

can 1 guide me regarding this,i new mr

as in example want details of student "test1" collection,studying in class "i".

your requirement seems be:

as in example want details of student "test1" collection,studying in class "i".

in order that, store classes student in student:

{     student_id: "xxxxx",     classes: ["i"], }, 

then can ask students information with:

db.students.find( { classes: "i" } ); 

without need slow , complex map reduce jobs. in general, should avoid map/reduce can't make use of indexes , can not run concurrently. need understand in mongodb operations done on one collection. there no such thing join, , trying emulate map/reduce bad idea. @ least can 2 queries:

// find students in class "i": ids = [];  db.classes.find( { class: "i" } ).foreach(function(e) { ids.push( e.student_id ) ; } ); // result, find of students information: db.students.find( { student_id: { $in: ids } } ); 

but recommend redesign schema , store classes each student. general hint, in mongodb store relation between documents on other side compared relational database.