How to $match and then accumulate within $group stage in MongoDB aggregation

Question

In one aggregation stage within my MongoDB view pipeline I am using $group. One of the things I'm doing within that $group stage is summing up an openBalance, like this:

  "openBalance": {
    "$sum": "$openBalance"
  },

This works.

However, I also need to get another value, that first matches to a condition, and then sums data. In other words, something like this:

  "customerResponsibleOpenBalance": {
    "$match": {
      "customerResponsible": true
    },
    "$sum": "$openBalance"
  },

However, I can't do this like this within a $group stage, because it results in this error:

customerResponsibleOpenBalance must specify one accumulator.

So how would I go about doing this in a different way? Are $facets my only option here?

Answer 1

$group works on a set of documents so you can only use accumulators to get a result. To get $sum conditionally you can use $cond operator inside of it, and add 0 if there's no match:

{
    $group: {
        _id: null,
        balanceForTrue: { $sum: { $cond: { if: { $eq: [ "$customerResponsible", true ] }, then: "$balance", else: 0  } } },
        balanceForFalse: { $sum: { $cond: { if: { $eq: [ "$customerResponsible", false ] }, then: "$balance", else: 0  } } }
    }
}