MongoDB汇总字段而无需事先知道所有字段

Question

在不事先掌握所有指标的情况下，如何计算以下指标的总和？我可以使用聚合框架或MapReduce完成此操作吗？

[
  {
   player_id: '123',
   timestamp: <sometime>,
   metrics: {
     points_per_game: 1,
     rebounds_per_game: 2,
     assist_per_game: 3,
   }
  },
  {
    player_id: '123',
    timestamp: <sometime>,
    metrics: {
      points_per_game: 1,
      rebounds_per_game: 2,
    }
  },
  {
    player_id: '345',
    timestamp: <sometime>,
    metrics: {
      points_per_game: 1,
      rebounds_per_game: 2,
      point_in_the_paint_per_game: 2
    }
  }
]

我想得到以下结果

[
 {
   player_id: '123',
   metrics: {
     points_per_game: 2,
     rebounds_per_game: 4,
     assist_per_game: 3,
   }
 },
 {
   player_id: '345',
   metrics: {
     points_per_game: 1,
     rebounds_per_game: 2,
     point_in_the_paint_per_game: 2
   }
 }
]

我无法执行以下操作，因为这将要求我了解每个指标：

db.stats.aggregate([
   {$group: {
     _id: {player: "$player_id"},
     points_per_game: { $sum: "$metrics.points_per_game"}
     ...
])

Answer 1

您可以尝试下面的聚合。

将对象转换为键值对数组，后跟 $unwind+$group 以按每个键进行分组并累加计数。返回命名键值对象的最后一步。

db.colname.aggregate([
  {"$addFields":{"metrics":{"$objectToArray":"$metrics"}}},
  {"$unwind":"$metrics"},
  {"$group":{
    "_id":{"id":"$player_id","key":"$metrics.k"},
    "count":{"$sum":"$metrics.v"}
  }},
  {"$group":{
    "_id":"$_id.id",
    "metrics":{"$mergeObjects":{"$arrayToObject":[[["$_id.key","$count"]]]}}
  }}
])