improved readability

app/models/exercise.rb

@@ -24,7 +24,10 @@ class Exercise < ActiveRecord::Base
   end
 
   def average_score
-    ActiveRecord::Base.connection.execute("SELECT AVG(score) AS average_score FROM (SELECT MAX(score) AS score FROM submissions WHERE cause = 'submit' AND exercise_id = '#{id}' GROUP BY user_id) AS maximum_scores")[0]['average_score'].to_f.round(2)
+    if submissions.exists?(cause: 'submit')
+      maximum_scores_query = submissions.select('MAX(score) AS maximum_score').where(cause: 'submit').group(:user_id).to_sql
+      self.class.connection.execute("SELECT AVG(maximum_score) AS average_score FROM (#{maximum_scores_query}) AS maximum_scores").first['average_score'].to_f
+    end
   end
 
   def duplicate(attributes = {})

app/views/exercises/statistics.html.slim

@@ -5,5 +5,5 @@ h1 = @exercise
   = row(label: ".#{scope}_submissions") do
     = "#{@exercise.submissions.send(scope).count} (#{t('.users', count: @exercise.submissions.send(scope).distinct.count(:user_id, :user_type))})"
 = row(label: '.average_score') do
-  p == @exercise.average_score ? t('shared.out_of', maximum_value: @exercise.maximum_score, value: @exercise.average_score) : empty
+  p == @exercise.average_score ? t('shared.out_of', maximum_value: @exercise.maximum_score, value: @exercise.average_score.round(2)) : empty
   p = progress_bar(@exercise.average_percentage)

app/views/sessions/destroy_through_lti.html.slim

@@ -11,5 +11,5 @@ h2 = t('shared.statistics')
   p = progress_bar(@submission.percentage)
 = row(label: '.final_submissions', value: @submission.exercise.submissions.final.distinct.count(:user_id, :user_type) - 1)
 = row(label: '.average_score') do
-  p == t('shared.out_of', maximum_value: @submission.exercise.maximum_score, value: @submission.exercise.average_score)
+  p == t('shared.out_of', maximum_value: @submission.exercise.maximum_score, value: @submission.exercise.average_score.round(2))
   p = progress_bar(@submission.exercise.average_percentage)