Add position attribute to relation between exercise collection and exercises

app/models/exercise.rb

@@ -14,7 +14,8 @@ class Exercise < ActiveRecord::Base
 
   has_and_belongs_to_many :proxy_exercises
   has_many :user_proxy_exercise_exercises
-  has_and_belongs_to_many :exercise_collections
+  has_many :exercise_collection_items
+  has_many :exercise_collections, through: :exercise_collection_items
   has_many :user_exercise_interventions
   has_many :interventions, through: :user_exercise_interventions
   has_many :exercise_tags

app/models/exercise_collection.rb

@@ -1,7 +1,8 @@
 class ExerciseCollection < ActiveRecord::Base
   include TimeHelper
 
-  has_and_belongs_to_many :exercises
+  has_many :exercise_collection_items
+  has_many :exercises, through: :exercise_collection_items
   belongs_to :user, polymorphic: true
 
   def exercise_working_times

app/models/exercise_collection_item.rb

@@ -0,0 +1,4 @@
+class ExerciseCollectionItem < ActiveRecord::Base
+  belongs_to :exercise_collection
+  belongs_to :exercise
+end