break out author subscriptions to its own migration

db/migrate/20161002153425_add_join_table_between_users_and_diary_entries.rb

@@ -9,12 +9,4 @@ class AddJoinTableBetweenUsersAndDiaryEntries < ActiveRecord::Migration
     add_index :diary_entry_subscriptions, [:diary_entry_id]
   end
 
-  def up
-    DiaryEntry.find_each do |diary_entry|
-      diary_entry.subscriptions.create(:user => diary_entry.user) unless diary_entry.subscribers.exists?(@user.id)
-    end
-  end
-
-  def down
-  end
 end

db/migrate/20161011010929_subscribe_authors_to_diary_entries.rb

@@ -0,0 +1,10 @@
+class SubscribeAuthorsToDiaryEntries < ActiveRecord::Migration
+  def up
+    DiaryEntry.find_each do |diary_entry|
+      diary_entry.subscriptions.create(:user => diary_entry.user) unless diary_entry.subscribers.exists?(diary_entry.user.id)
+    end
+  end
+
+  def down
+  end
+end