MongoDB聚合框架的绝对价值

问题描述

我正在使用MongoDB聚合框架，并且需要取一个金额字段的绝对值，我在项目部分和组部分中都使用了该值，例如:

I'm using the MongoDB aggregation framework and need to take the absolute value of an amount field, which I use in both the project portion and the group portion, ex:

'$project' => {
          'amount' => 1,
          '_id' => 0
        }

....

    '$group' => {
      'total' => {'$sum' => '$amount'}
    }

在这种情况下，如何获取金额"字段的绝对值?我无法在文档中找到它(也许它不可用?)

How do you take the absolute value of the 'amount' field in this case? I wasn't able to find it in the docs (maybe it's not available?)

推荐答案

它不是直接可用的，但是您可以使用$cond运算符和$subtract在$project中使用$subtract来做到这一点(作为JavaScript对象) ):

It's not directly available, but you can do it using a $cond operator and a $subtract within a $project like this (as a JavaScript object):

{ $project: {
  amount: {
    $cond: [
      { $lt: ['$amount', 0] },
      { $subtract: [0, '$amount'] },
      '$amount'
    ]
}}}

因此，如果使用amount < 0，则使用0 - amount，否则直接使用amount.

So if amount < 0, then 0 - amount is used, otherwise amount is used directly.

更新

从MongoDB的 3.2 版本开始，您可以使用新的 $abs 聚合表达式运算符可直接执行此操作:

As of the 3.2 release of MongoDB, you can use the new $abs aggregation expression operator to do this directly:

{ $project: { amount: { $abs: '$amount' } }

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