MongoDB攻击多组

我有数百万张这样的唱片：

{
    "_id" : ObjectId("568adba57faad8f22ff38a07"),
    "temperature" : 27,
    "elapsedTime" : 27,
    "stationId" : 28,
    "beaconId" : 238,
    "beaconClass" : 1,
    "batteryLevel" : 61
}

{"stationId":1, "beaconClass":1, total_time: summary_of_elapsedTime_in_station_for_this_beaconClass}
{"stationId":1, "beaconClass":2, total_time: summary_of_elapsedTime_in_station_for_this_beaconClass}
...
...
{"stationId":2, "beaconClass":1, total_time: summary_of_elapsedTime_in_station_for_this_beaconClass}
{"stationId":2, "beaconClass":2, total_time: summary_of_elapsedTime_in_station_for_this_beaconClass}
{"stationId":2, "beaconClass":3, total_time: summary_of_elapsedTime_in_station_for_this_beaconClass}
...
...
// so on

我想将我的记录分组如下：

{
    "_id" : ObjectId("568adba57faad8f22ff38a07"),
    "temperature" : 27,
    "elapsedTime" : 27,
    "stationId" : 28,
    "beaconId" : 238,
    "beaconClass" : 1,
    "batteryLevel" : 61
}

{"stationId":1, "beaconClass":1, total_time: summary_of_elapsedTime_in_station_for_this_beaconClass}
{"stationId":1, "beaconClass":2, total_time: summary_of_elapsedTime_in_station_for_this_beaconClass}
...
...
{"stationId":2, "beaconClass":1, total_time: summary_of_elapsedTime_in_station_for_this_beaconClass}
{"stationId":2, "beaconClass":2, total_time: summary_of_elapsedTime_in_station_for_this_beaconClass}
{"stationId":2, "beaconClass":3, total_time: summary_of_elapsedTime_in_station_for_this_beaconClass}
...
...
// so on

我可以按stationId、beaconId或class进行分组。但当我尝试按多个列分组时，出现了错误

---

如何为此创建聚合查询？

要对多个字段进行分组，需要为_id值指定聚合键。看看$group

请尝试以下查询：

db.collection.aggregate([
  {  
    $group: {
      _id: {stationId: "$stationId", beaconClass: "$beaconClass"}, 
      elapsedTime: {$sum: "$elapsedTime"}
    } 
  }
])

如果需要将收到的字段投影到另一个架构，可以利用聚合中的运算符：

{
    $project: { _id: 0, stationId: "$_id.stationId", beaconClass: "$_id.beaconClass", elapsedTime: 1 } 
} 

---

它起作用了！真不敢相信这么简单。谢谢。你能给我解释一下为什么我们没有使用多个

$group

键吗？它怎么能处理得这么简单呢？不要从下面的查询开始回答问题，而是试着解释一下你的查询在做什么以及它为什么工作。此外，您不需要在此处使用

$project

阶段，因为它会导致性能下降