Retrieve user statistics in an execution environment for all exercises at once

app/controllers/execution_environments_controller.rb

@@ -55,14 +55,47 @@ class ExecutionEnvironmentsController < ApplicationController
     """
   end
 
+  def user_query
+    """
+    SELECT
+      id AS exercise_id,
+      COUNT(DISTINCT user_id) AS users,
+      AVG(score) AS average_score,
+      MAX(score) AS maximum_score,
+      CASE
+        WHEN MAX(score)=0 THEN 0
+        ELSE 100 / MAX(score) * AVG(score)
+      END AS percent_correct,
+      SUM(submission_count) / COUNT(DISTINCT user_id) AS average_submission_count
+    FROM
+      (SELECT e.id,
+              s.user_id,
+              MAX(s.score) AS score,
+              COUNT(s.id) AS submission_count
+       FROM submissions s
+       JOIN exercises e ON e.id = s.exercise_id
+       WHERE e.execution_environment_id = #{@execution_environment.id}
+       GROUP BY e.id,
+                s.user_id) AS inner_query
+    GROUP BY id;
+    """
+  end
+
   def statistics
     working_time_statistics = {}
+    user_statistics = {}
+
     ActiveRecord::Base.connection.execute(working_time_query).each do |tuple|
       working_time_statistics[tuple["exercise_id"].to_i] = tuple
     end
 
+    ActiveRecord::Base.connection.execute(user_query).each do |tuple|
+      user_statistics[tuple["exercise_id"].to_i] = tuple
+    end
+
     render locals: {
-      working_time_statistics: working_time_statistics
+      working_time_statistics: working_time_statistics,
+      user_statistics: user_statistics
     }
   end
 

app/models/exercise.rb

@@ -34,7 +34,7 @@ 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.sub('$1', id.to_s)
+      maximum_scores_query = submissions.select('MAX(score) AS maximum_score').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
     else 0 end
   end

app/views/execution_environments/statistics.html.slim

@@ -8,14 +8,15 @@ h1 = @execution_environment
           th.header = t(title)
     tbody
       - @execution_environment.exercises.each do |exercise|
-        - average_score = exercise.average_score
+        - us = user_statistics[exercise.id]
+        - if not us then us = {"users" => 0, "average_score" => 0.0, "maximum_score" => 0, "percent_correct" => nil, "average_submission_count" => 0}
         - wts = working_time_statistics[exercise.id]
         - if wts then average_time = wts["average_time"] else 0
         tr
           td = link_to exercise.title, controller: "exercises", action: "statistics", id: exercise.id
-          td = exercise.users.distinct.count
-          td = average_score
-          td = exercise.maximum_score
-          td = 100 / exercise.maximum_score * average_score
-          td = exercise.average_number_of_submissions
+          td = us["users"]
+          td = us["average_score"]
+          td = us["maximum_score"]
+          td = us["percent_correct"]
+          td = us["average_submission_count"]
           td = average_time