mongodb在aggregate中怎么统计总数

[size=medium]在aggregate只能通过$group中的$sum来进行统计，如果sales表中有下面数据：[/size]

{ "_id" : { "month" : 3, "day" : 15, "year" : 2014 }, "totalPrice" : 50, "averageQuantity" : 10, "count" : 1 }
{ "_id" : { "month" : 4, "day" : 4, "year" : 2014 }, "totalPrice" : 200, "averageQuantity" : 15, "count" : 2 }
{ "_id" : { "month" : 3, "day" : 1, "year" : 2014 }, "totalPrice" : 40, "averageQuantity" : 1.5, "count" : 2 }

[size=medium]如果要得到count总记录数该怎么做呢？
我们可以通过：[/size]

db.sales.aggregate(
   [
      {
        $group : {
           _id : null,
           totalPrice: { $sum: { $multiply: [ "$price", "$quantity" ] } },
           averageQuantity: { $avg: "$quantity" },
           count: { $sum: 1 }
        }
      }
   ]
)

[size=medium]即可得到，id:null这个非常关键
得到如下结果[/size]：

{ "_id" : null, "totalPrice" : 290, "averageQuantity" : 8.6, "count" : 5 }         

[size=medium]java代码可以通过如下[/size]

	DBObject count = new BasicDBObject("$sum",1);
			DBObject groupFileds = new BasicDBObject("_id", null);
			groupFileds.put("count", count);
			DBObject group = new BasicDBObject("$group", groupFileds);
  AggregationOutput output = collection.aggregate(Arrays.asList(group));
List<DBObject> dbObjects = (List<DBObject>) output.results()；
int count = (int) dbObjects.get(0).get("count");

这里干嘛不直接用mongodb的count，这是因为如果在这个统计中如果有根据地理空间距离查找的要求，那么就不满足要求了，所以改用aggregate进行统计是个不错的选择 阅读更多