Refactor average worktime and add it to exercise statistics per user

app/models/exercise.rb

@@ -42,26 +42,39 @@ class Exercise < ActiveRecord::Base
     return user_count == 0 ? 0 : submissions.count() / user_count.to_f()
   end
 
+  def user_working_time_query
+    """
+      SELECT user_id,
+             sum(working_time_new) AS working_time
+      FROM
+        (SELECT user_id,
+                CASE WHEN working_time >= '0:30:00' THEN '0' ELSE working_time END AS working_time_new
+         FROM
+            (SELECT user_id,
+                    id,
+                    (created_at - lag(created_at) over (PARTITION BY user_id
+                                                        ORDER BY id)) AS working_time
+            FROM submissions
+            WHERE exercise_id=#{id}) AS foo) AS bar
+      GROUP BY user_id
+    """
+  end
+
   def average_working_time
     self.class.connection.execute("""
       SELECT avg(working_time) as average_time
       FROM
-        (SELECT user_id,
-                sum(working_time_new) AS working_time
-         FROM
-           (SELECT user_id,
-                   CASE WHEN working_time >= '0:30:00' THEN '0' ELSE working_time END AS working_time_new
-            FROM
-              (SELECT user_id,
-                      id,
-                      (created_at - lag(created_at) over (PARTITION BY user_id
-                                                          ORDER BY id)) AS working_time
-               FROM submissions
-               WHERE exercise_id=#{id}) AS foo) AS bar
-         GROUP BY user_id) AS baz;
+        (#{user_working_time_query}) AS baz;
     """).first['average_time']
   end
 
+  def average_working_time_for(user_id)
+    self.class.connection.execute("""
+      #{user_working_time_query}
+      HAVING user_id = #{user_id}
+    """).first['working_time']
+  end
+
   def duplicate(attributes = {})
     exercise = dup
     exercise.attributes = attributes

app/views/exercises/statistics.html.slim

@@ -23,5 +23,4 @@ h1 = @exercise
             td = "#{user.name} (#{user.email})"
             td = submissions.maximum('score')
             td = submissions.count('id')
-            - seconds = submissions.maximum('created_at') - submissions.minimum('created_at')
-            td = "#{distance_of_time_in_words(seconds)} (#{seconds})"
+            td = @exercise.average_working_time_for(user.id) or 0