How to concat string and a substr?

[Cyril_Auburtin1 (Cyril Auburtin)]

I tried this below but got: { MongoServerError: PlanExecutor error during aggregation :: caused by :: $concat only supports strings, not objectId and I don’t know what to do

        $group: {
          _id: {
            $concat: [
              { $substr: ['$period', 0, groupByLength[options.groupBy]] },
              '|',
              '$clientId',
              '|',
              '$endpoint',
            ]
          },
          count: { $sum: '$count' },
          points: { $avg: { $multiply: ['$count', '$points'] } },
        }```

[aneroid (Anirudh Dutt)]

'$clientId' is probably an ObjectId. If so, use $toString, as { $toString: "$clientId" }.

Provide some example documents and expected outcome. You can also use Mongo Playground to create a reproducible example.

Btw, to create a unique _id for $group with multiple fields, you don’t have to concatenate the different fields. You can directly use each of the fields, like this:

{
  $group: {
    _id: {
      // multiple fields
      periodPart: { $substr: ["$period", 0, groupByLength[options.groupBy]] },
      clientId: "$clientId",  // no need to convert clientId to a string
      endpoint: "$endpoint"
    },
    count: { $sum: "$count" },
    points: { $avg: { $multiply: ["$count", "$points"] } }
  }
}