如何使用mongodb和NumberLong计算平均值

Question

我试图使用mongodb java驱动程序计算集合的平均值,如下所示:

DBObject condition = 
    new BasicDBObject("pluginIdentifier", plugin.getIdentifier());

DBObject initial = new BasicDBObject();

initial.put("count", 0);
initial.put("totalDuration", 0);
String reduce = "function(duration, out) { out.count++; 
    out.totalDuration+=duration.floatApprox; }";
String finalize = "function(out) { out.avg = out.totalDuration.floatApprox / 
    out.count; }";

DBObject avg = durationEntries.group(
    new BasicDBObject("pluginIdentifier", true), 
    condition, initial, reduce, finalize);

System.out.println(avg);

"duration"是NumberLong(在java中,它是Long,可能是java驱动程序转换它).经过一些搜索我发现,为了提取数字,使用.floatApprox是一种方法,这也适用于mongodb控制台:

> db.DurationEntries.findOne().duration.floatApprox
5

但是,运行上面的java代码不会计算平均值,而是返回它

[{"pluginIdentifier":"dummy", "count":7.0, "totalDuration":NaN, "avg":NaN}]

我尝试了几种变体,有和没有.floatApprox,但到目前为止只能获得一些奇怪的字符串连接.

我的问题是:我做错了什么/我应该如何计算一个NumberLong列的平均值？

Answer 1

如果你遇到map/reduce问题,你可能应该进入mongodb控制台,在那里进行操作,然后将其转换为你的驱动程序.

举例来说,以下文件:

db.tasks.find()
{ "_id" : ObjectId("4dd51c0a3f42cc01ab0e6506"), "duration" : 10, "name" : "StartProcess", "date" : "20110501" }
{ "_id" : ObjectId("4dd51c0e3f42cc01ab0e6507"), "duration" : 11, "name" : "StartProcess", "date" : "20110502" }
{ "_id" : ObjectId("4dd51c113f42cc01ab0e6508"), "duration" : 12, "name" : "StartProcess", "date" : "20110503" }

您可以编写mapReduce来计算StartProcess的平均持续时间,如下所示:

m = function (){
  emit( this.name , { totalDuration : this.duration , num : 1 } );
};

r = function (name, values){
  var n = {totalDuration : 0, num : 0};
  for ( var i=0; i<values.length; i++ ){
    n.totalDuration += values[i].totalDuration;
    n.num += values[i].num;
  }
  return n;
};

f = function(who, res){
  res.avg = res.totalDuration / res.num;
  return res;
};

然后,假设您使用的是MongoDB 1.7或更高版本:

db.tasks.mapReduce( m, r, { finalize : f, out : {inline : 1} });

会给你以下答案:

"results" : [
  {
    "_id" : "StartProcess",
      "value" : {
        "totalDuration" : 33,
        "num" : 3,
        "avg" : 11
      }
  }
]

如果这没有帮助,您可以发布您的地图功能和文档结构.