We don't need to disable transaction in this case

db/migrate/20160804142904_add_ci_config_file_to_project.rb

-# See http://doc.gitlab.com/ce/development/migration_style_guide.html
-# for more information on how to write migrations for GitLab.
-
 class AddCiConfigFileToProject < ActiveRecord::Migration
-  include Gitlab::Database::MigrationHelpers
-
   DOWNTIME = false
 
-  disable_ddl_transaction!
-
   def change
     add_column :projects, :ci_config_file, :string
   end