Absolute value with MongoDB aggregation framework

问题

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'}
    }

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?)

回答1:

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'
    ]
}}}

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

UPDATE

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' } }

来源：https://stackoverflow.com/questions/17561804/absolute-value-with-mongodb-aggregation-framework