学步园

Map:

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

Reduce:

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

执行：

result=db.runCommand({  
    mapreduce:"students",  
    map:map,  
    reduce:reduce,  
    out:"students_result"  
}); 

摘自：http://chenzhou123520.iteye.com/blog/1637672