Aggregation on $group and $count query should skip the null values while returning count

For the below query, the first aggregation will result null value and then it is counted as 1 in second stage, how can we skip counting the null value from the distinct

db.party.aggregate([
{ $group: { _id: “$party1.partyRole” } },
{ $count: “count” }
]);

stage1: { _id: “$party1.partyRole” } => _id: null

stage2: “count” => 1

But from the stage one, if there is only null value then the count should skip it and show as 0 instead of 1.

You can keep your current pipeline. Just add a $unionWith stage at the end to append a count: 0 document. Then, perform a $limit: 1 to pick the 1st document. If there is non-null result, it will be picked up as the first document and return the count result. If there is only null result, it will pick up the count: 0 document we appended.

db.collection.aggregate([
  {
    $group: {
      _id: "$party1.partyRole"
    }
  },
  {
    "$match": {
      _id: {
        $ne: null
      }
    }
  },
  {
    "$count": "count"
  },
  {
    "$unionWith": {
      "coll": "collection",
      "pipeline": [
        {
          "$documents": [
            {
              "count": 0
            }
          ]
        }
      ]
    }
  },
  {
    "$limit": 1
  }
])

Mongo Playground with only null result
Mongo Playground with non-null result