多維查找最大值MongoDB中

Question

，如果我有這個集合

{ "humidity" : 96.5812, "temperature" : 10.5006 } 
{ "humidity" : 97.1184, "temperature" : 10.2808 } 
{ "humidity" : 96.2882, "temperature" : 8.4493 } 
{ "humidity" : 97.8266, "temperature" : 7.4481 } 
{ "humidity" : 98.9255, "temperature" : 7.2772 } 
{ "humidity" : 99.4628, "temperature" : 7.3993 } 
{ "humidity" : 99.4383, "temperature" : 7.4237 } 
{ "humidity" : 99.6825, "temperature" : 7.1307 } 
{ "humidity" : 99.5116, "temperature" : 6.1539 } 
{ "humidity" : 99.8779, "temperature" : 5.4701 } 

我怎樣才能得到最大，並與MapReduce的溫度的最小值？

Answer 1

// Simple map function - just returns the temperature value 
// for the record 
var map = function() { 
    emit('temperature', this.temperature); 
}; 

// A reduce function to find the minimum value in the reduced set 
var reduce_min = function(key, values) { 
    var min = values[0]; 
    values.forEach(function(val) { 
     if (val < min) min = val; 
    }) 
    return min; 
}; 

// A reduce function to find the maximum value in the reduced set 
var reduce_max = function(key, values) { 
    var max = values[0]; 
    values.forEach(function(val){ 
     if (val > max) max = val; 
    }) 
    return max; 
} 

// Use the mapReduce function to get the min and max 
var min = db.temp.mapReduce(map, reduce_min, {out:{inline:1}}).results[0].value; 
var max = db.temp.mapReduce(map, reduce_max, {out:{inline:1}}).results[0].value; 
print("Min: " + min + ", max: " + max);