avgFingerprint

db.rares_news_collection.aggregate([
  {
    $project: {
      _id: 0,
      fingerprintSize: {
        $cond: [
          { $isArray: "$fingerprints" },
          { $size: "$fingerprints" },
          0
        ]
      }
    }
  },
  {
    $group: {
      _id: null,
      avgFingerprintSize: { $avg: "$fingerprintSize" }
    }
  },
  {
    $project: {
      _id: 0,
      avgFingerprintSize: 1
    }
  }
])