文字列をobjectIDforeignFieldにルックアップするための回避策が必要です

4.0の新機能：https：//docs.mongodb.com/manual/reference/operator/aggregation/toObjectId/

// Define stage to add convertedId field with converted _id value

idConversionStage = {
   $addFields: {
      convertedId: { $toObjectId: "$_id" }
   }
};

// Define stage to sort documents by the converted qty values

sortStage = {
   $sort: { "convertedId": -1 }
};


db.orders.aggregate( [
   idConversionStage,
   sortStage
])