Add user to exercise_collection

app/models/exercise_collection.rb

@@ -1,6 +1,7 @@
 class ExerciseCollection < ActiveRecord::Base
 
   has_and_belongs_to_many :exercises
+  belongs_to :user, polymorphic: true
 
   def to_s
     "#{I18n.t('activerecord.models.exercise_collection.one')}: #{name} (#{id})"