transitionally downgraded to Rails 4.1 since activerecord-jdbcpostgresql-adapter is not fully compatible with Active Record 4.2

app/models/exercise.rb

@@ -25,7 +25,7 @@ class Exercise < ActiveRecord::Base
 
   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
+      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
   end