mongodb mapreduce学习

for( var i = 1; i < 10001; i++){
 db.students.insert({classid:i%400, age:i%100, name:'Tom'+i})
}

db.students.insert({classid:1, age:14, name:'Tom'})
db.students.insert({classid:1, age:12, name:'Jacky'})
db.students.insert({classid:2, age:16, name:'Lily'})
db.students.insert({classid:2, age:9, name:'Tony'})
db.students.insert({classid:2, age:19, name:'Harry'})
db.students.insert({classid:2, age:13, name:'Vincent'})
db.students.insert({classid:1, age:14, name:'Bill'})
db.students.insert({classid:2, age:17, name:'Bruce'})


m = function() {
    emit(this.classid, 1) 
}


r = function(key, values) {
     var x = 0;
     values.forEach(
        function(v) { x += v }
     );
    return x;
 }

 res = db.runCommand({
    mapreduce:"students",
     map:m,
    reduce:r,
    out:"students_res"
 });

db.students_res.find()

m = function() {
    emit(this.classid, {cnt:1,name:this.name}) 
}


r = function(key, values) {
     var x = 0;
     var names="";
     values.forEach(
        function(v)
        {
            x += v.cnt;
            names+=v.name;
        }
     );
    return {classid:key,cnt:x,names:names};
 }

 res = db.runCommand({
    mapreduce:"students",
     map:m,
    reduce:r,
    out:"students_res"
 });

db.students_res.find()

f = function(key, value) { return {classid:key, count:value}; }

res = db.runCommand({
    mapreduce:"students",
    map:m,
    reduce:r,
    out:"students_res",
    finalize:f
});

db.students_res.find()

res = db.runCommand({
     mapreduce:"students",
     map:m,
     reduce:r,
     out:"students_res",
     finalize:f,
     query:{age:{$lt:10}}
 });

db.students_res.find()