Mongo减去组聚合 [英] Mongo subtract in group aggregation

问题描述

我有一个mongo查询，我需要分组，而不是从最大值中减去最小值，并且这样做有问题.

I have a mongo query and I need to group and than subtract min value from max value and I have issues doing that.

这是我的查询:

{
    $group : {
        _id :  {
            type: "$type",
            id : "$id"
            },
        sent : {$subtract: [{$max : "$value"}, {$min : "$value"}]}
    }
}

但是我收到此错误消息:

But I get this error message:

$ subtract累加器是一元运算符

The $subtract accumulator is a unary operator

我想知道如何在小组赛中减去.

I would like to know how do I subtract inside the group stage.

推荐答案

可以尝试以下查询:

db.[collection].aggregate([   
{
        $group : {
            _id :  {
                type: "$type",
                id : "$id"
                }, 
            maxValue : {$max : "$value"} , 
            minValue : {$min : "$value"}
        }
    },
    {
        $group : {
            _id :  {
                type: "$_id.type",
                id : "$_id.id"
                }, 
            sent : {$subtract: [ "$maxValue", "$minValue" ]}
        }
    }
 ]) 

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