Referencing another field of subdocument in `$elemMatch`

Question

I'm trying to perform an $elemMatch in a $match aggregation stage where I want to find if there is a document in an array (commitments) of subdocuments whose property tracksThisWeek is smaller than its frequency property, but I'm not sure how can I reference another field of the subdocument in question, I came up with:

{
    $match: {
        commitments: {
            $elemMatch: {
                tracksThisWeek: {
                    $lt: '$frequency',
                },
            },
        },
    },
},

I have a document in the collection that should be returned from this aggregation but isn't, any help is appreciated :)

Answer 1

This can't be done, you can't reference any fields in the query language, What you can do is use $expr with aggregation operators, like this:

db.collection.aggregate([
  {
    $match: {
      $expr: {
        $gt: [
          {
            $size: {
              $filter: {
                input: "$commitments",
                cond: {
                  $lt: [
                    "$$this.tracksThisWeek",
                    "$$this.frequency"
                  ]
                }
              }
            }
          },
          0
        ]
      }
    }
  }
])

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