这种聚合可以在没有$function或$accumulator的Mongo上完成吗？(某种地图缩减)

Question

我正在尝试将一个mapReduce操作转换成一个聚合管道，但是在这一步中我被阻塞了。我知道我可以使用标题中提到的运算符编写一个自定义函数，但理想情况下，我希望不用自定义聚合器来解决它。

由此：

{
field1: 'A',
field2: 'B',
field3: 'C',
statuses:[
   {
       key:"RECEIVED"
       value:20
   },{
      key:"NOT_DELIVERED"
      value:54
   },{
      key:"RECEIVED"
      value:30
   },{
      key:"RECEIVED"
      value:20
   },{
      key:"DELIVERED"
      value:58
   },{
      key:"DELIVERED"
      value:2
   }]}

我想要这个：(对于每个键，它们的值之和)

{
field1: 'A',
field2: 'B',
field3: 'C',
statuses:[
   {
      key:"NOT_DELIVERED"
      value:54
   },{
      key:"RECEIVED"
      value:70
   },{
      key:"DELIVERED"
      value:60
   }]}

Answer 1

你可以用这个：

db.collection.aggregate([
   {
      $set: {
         statuses: {
            $map: {
               input: { $setUnion: "$statuses.key" },
               as: "key",
               in: {
                  key: "$$key",
                  status: {
                     $filter: {
                        input: "$statuses",
                        as: "status",
                        cond: { $eq: ["$$status.key", "$$key"] }
                     }
                  }
               }
            }
         }
      }
   },
   {
      $set: {
         statuses: {
            $map: {
               input: "$statuses",
               in: { key: "$$this.key", value: { $sum: "$$this.status.value" } }
            }
         }
      }
   }
])

如果您喜欢$group，那么使用这个：

db.collection.aggregate([
   { $unwind: "$statuses" },
   {
      $group: {
         _id: { field1: "$field1", field2: "$field2", field3: "$field3", status: "$statuses.key" },
         value: { $sum: "$statuses.value" }
      }
   },
   {
      $group: {
         _id: { field1: "$_id.field1", field2: "$_id.field2", field3: "$_id.field3" },
         statuses: { $push: { key: "$_id.status", value: "$value" } }
      }
   },
   { $replaceRoot: { newRoot: { $mergeObjects: ["$$ROOT", "$_id"] } } },
   { $unset: "_id" }
])