Absolute value with MongoDB aggregation framework

Question

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:

Answer 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' } }