Split migration into adding column and adding FK

ee/db/migrate/20181123090058_add_parent_to_epic.rb

@@ -7,23 +7,11 @@ class AddParentToEpic < ActiveRecord::Migration[5.0]
 
   DOWNTIME = false
 
-  disable_ddl_transaction!
-
   def up
-    add_column :epics, :parent_id, :integer unless parent_id_exists?
-    add_concurrent_foreign_key :epics, :epics, column: :parent_id, on_delete: :cascade
-    add_concurrent_index :epics, :parent_id
+    add_column :epics, :parent_id, :integer
   end
 
   def down
-    remove_foreign_key_without_error(:epics, column: :parent_id)
-    remove_concurrent_index(:epics, :parent_id)
-    remove_column(:epics, :parent_id) if parent_id_exists?
-  end
-
-  private
-
-  def parent_id_exists?
-    column_exists?(:epics, :parent_id)
+    remove_column :epics, :parent_id
   end
 end

ee/db/migrate/20181123100058_add_parent_epic_fk.rb

+# frozen_string_literal: true
+
+# See http://doc.gitlab.com/ce/development/migration_style_guide.html
+# for more information on how to write migrations for GitLab.
+class AddParentEpicFk < ActiveRecord::Migration[5.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def up
+    add_concurrent_foreign_key :epics, :epics, column: :parent_id, on_delete: :cascade
+    add_concurrent_index :epics, :parent_id
+  end
+
+  def down
+    remove_foreign_key :epics, column: :parent_id
+    remove_concurrent_index :epics, :parent_id
+  end
+end