如何使用聚合框架在mongoDB中计算加权平均值？ [英] How do I calculate a weighted average in mongoDB using aggregation framework?

问题描述

我需要计算一组文档的加权平均值。每个文档在单独的字段中都包含权重和值。
这是一个示例：
我有以下两个代表销售交易的文档。

I need to calculate weighted average over a set of documents. Each document contains both a weight and a value in a separate field. Here is an example: I have following 2 documents representing a sales transaction.

{
    quantity : 3,
    price : 2
}

{
    quantity : 9,
    price : 6
}

我想找到两个交易的平均价格。这是加权平均值，其中权重是数量，价值是价格。可以通过

I want to find the average price for both transactions. This is a weighted average where weight is the quantity and value is the price. This can be calculated by

AveragePrice =（3 * 2 + 9 * 6）/（3 + 9）来计算

如何使用聚合框架执行此计算？

How do I perform this calculation using aggregation framework?

推荐答案

为此，您应该首先计算所得比率的分子（加权和）和分母（权重之和）。之后，您只需将它们彼此分开：

To do so you should first calculate numerator (weighted sum) and denominator (sum of weights) of the resulting ratio. After that you'll only need to divide one by another:

db.collection.aggregate({
  $group : {
     _id : 'weighted average', // build any group key ypo need
     numerator: { $sum: { $multiply: [ "$price", "$quantity" ] } },
     denominator: { $sum: "$quantity" }
  }
}, {
  $project: {
    average: { $divide: [ "$numerator", "$denominator" ] }
  }
})

有关更多信息，请参见聚合管道文档。

For more info see Aggregation Pipeline documentation.

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