Add position attribute to relation between exercise collection and exercises

db/migrate/20180703125302_create_exercise_collection_items.rb

@@ -0,0 +1,13 @@
+class CreateExerciseCollectionItems < ActiveRecord::Migration
+  def up
+    rename_table :exercise_collections_exercises, :exercise_collection_items
+    add_column :exercise_collection_items,  :position, :integer, default: 0, null: false
+    add_column :exercise_collection_items,  :id, :primary_key
+  end
+
+  def down
+    remove_column :exercise_collection_items,  :position
+    remove_column :exercise_collection_items,  :id
+    rename_table :exercise_collection_items, :exercise_collections_exercises
+  end
+end