H5.3 Mongo course. Aggregate and group function

Aggregate

db.grades.aggregate([
   {$unwind: "$scores"}, 
   {$match: 
       {'scores.type': "homework"}},
   {$group: 
       {_id: {student_id: "$student_id", 
              class_id: "$class_id",
              scorestype: "$scores.type",
             }, 
       avg_scores: {$avg:"$scores.score"}
   }},
  /* {$group:
       {_id: {class: "$_id.class_id"}, 
       average: {$avg: "$avg_scores"}}

   },
   {$sort: {'average': 1}} */  
])