MongoDB group aggregation with count and sort

query.rb

q = [ 
  { "$match" => { "created_at" => { "$gte" => 60.days.ago, "$lte" => 0.days.ago }}}, 
  { "$group" => { _id: { 
                    'year' => {'$year' => "$created_at"},
                    'month' => {'$month' => "$created_at"},
                    'day' => {'$dayOfMonth' => "$created_at"},
                    "errorable_type" => "$errorable_type", 
                    "error_class" => "$error_class"
                  },
                  count: { "$sum" =>  1 }
                } 
  }, 
  { "$sort" => { "_id.year" => -1, "_id.month" => -1, "_id.day" => -1, "count" => -1} }
]

result = TaskError.collection.aggregate( q )

start = Time.now; last_date=nil;result.each{|r| date = "#{r.dig('_id','year')}-#{r.dig('_id','month')}-#{r.dig('_id','day')}"; if date != last_date; puts "\n\n#{date}\n\n"; last_date = date; end; puts " #{r['count']} => #{r['_id']}"};puts Time.now - start