MongoDB:通过计算每个字段的平均值将对象数组简化为单个对象 [英] MongoDB: Reduce array of objects into a single object by computing the average of each field

问题描述

我使用MongoDB聚合API每天聚合一些数据. 这种聚合的结果具有以下格式:

I use the MongoDB aggregation API to aggregate some data daily. The result of this aggregation is of this format:

[
  {
    aggDate: '2019-05-23',
    results: [
      {
        foo: 0.58,
        bar: 0.42
      }, {
        foo: 0.32,
        bar: 0.98
      }
    ]
  }
]

关于日期的聚合是可以的，但是现在我想聚合results数组中的对象.

The aggregation on the date is fine, but now I would like to aggregate the objects in the results array.

此聚合的结果应采用以下格式:

The result of this aggregation should be of the following format:

[
  {
    aggDate: '2019-05-23',
    result: {
      foo: 0.45 // avg of all the `foo`, here: (0.58 + 0.32) / 2
      bar: 0.7 // avg of all the `bar`, here: (0.42 + 0.98) / 2
    }
  }
]

我的问题是键foo和bar可以更改/可以在results对象中添加新字段.为了避免每次发生查询时都重新编码，我想使用某种通用的方式对MongoDB说

My problem here is that the keys foo and bar can change/new fields can be added in results objects. To avoid recoding the query each time it occurs, I want to use some generic way to say to MongoDB

采用此对象数组并将其简化为单个对象，其中每个值都是所有对象中同一字段的平均值.

Take this array of objects and reduce it into a single object where each value is the average of the same field in all objects.

我知道$reduce运算符存在于MongoDB中，但是我不知道如何使用它，甚至不确定它是否可以在这里为我提供帮助.

I know the $reduce operator exists in MongoDB but I can't figure out how to use it and I am not even sure if it can help me here.

推荐答案

您应运行 $ unwind 并使用 $ group 阶段.您还需要 $ arrayToObject 和 $ objectToArray 以使用动态键. $ reduce 在这里不是选项，因为键是unknwon

You should run $unwind and aggregate the data using $group stages. You also need $arrayToObject and $objectToArray to work with dynamic keys. $reduce is not an option here since keys are unknwon

db.col.aggregate([
    {
        $project: {
            aggDate: 1,
            results: {
                $map: { input: "$results", in: { $objectToArray: "$$this" } }
            }
        }
    },
    {
        $unwind: "$results"
    },
    {
        $unwind: "$results"
    },
    {
        $group: {
            _id: { aggDate: "$aggDate", k: "$results.k" },
            sum: { $sum: "$results.v" },
            count: { $sum: 1 }
        }
    },
    {
        $project: {
            _id: 1,
            v: { $divide: [ "$sum", "$count" ] }
        }
    },
    {
        $group: {
            _id: "$_id.aggDate",
            results: { $push: { k: "$_id.k", v: "$v" } }
        }
    },
    {
        $project: {
            _id: 0,
            aggDate: "$_id",
            results: { $arrayToObject: "$results" }
        }
    }
])

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