Move method to model

app/models/exercise_collection.rb

@@ -1,8 +1,17 @@
 class ExerciseCollection < ActiveRecord::Base
+  include TimeHelper
 
   has_and_belongs_to_many :exercises
   belongs_to :user, polymorphic: true
 
+  def average_working_time
+    working_times = {}
+    exercises.each do |exercise|
+      working_times[exercise.id] = time_to_f exercise.average_working_time
+    end
+    working_times.values.reduce(:+) / working_times.size
+  end
+
   def to_s
     "#{I18n.t('activerecord.models.exercise_collection.one')}: #{name} (#{id})"
   end