Push statistics features to model

app/models/exercise.rb

@@ -23,14 +23,19 @@ class Exercise < ActiveRecord::Base
   validates :token, presence: true, uniqueness: true
 
   def average_percentage
-    (average_score/ maximum_score * 100).round if average_score
+    (average_score / maximum_score * 100).round if average_score
   end
 
   def average_score
     if submissions.exists?(cause: 'submit')
       maximum_scores_query = submissions.select('MAX(score) AS maximum_score').where(cause: 'submit').group(:user_id).to_sql.sub('$1', id.to_s)
       self.class.connection.execute("SELECT AVG(maximum_score) AS average_score FROM (#{maximum_scores_query}) AS maximum_scores").first['average_score'].to_f
-    end
+    else 0 end
+  end
+
+  def average_working_time
+    working_time_query = submissions.select('MAX(created_at) - MIN(created_at) AS time').group(:user_id).to_sql.sub('$1', id.to_s)
+    self.class.connection.execute("SELECT AVG(time) AS average_time FROM (#{working_time_query}) AS working_times").first['average_time']
   end
 
   def duplicate(attributes = {})