```ruby
class AddStageIdForeignKeyToBuilds < ActiveRecord::Migration
  include Gitlab::Database::MigrationHelpers

  DOWNTIME = false

  disable_ddl_transaction!

  def up
    unless index_exists?(:ci_builds, :stage_id)
      add_concurrent_index(:ci_builds, :stage_id)
      add_concurrent_foreign_key(:ci_builds, :ci_stages, column: :stage_id, on_delete: :cascade)
    end
  end

  def down
    if index_exists?(:ci_builds, :stage_id)
      remove_foreign_key(:ci_builds, column: :stage_id)
      remove_concurrent_index(:ci_builds, :stage_id)
    end
  end
end
```

Check foreign keys in migration in separate conditional
user
```ruby
class AddStageIdForeignKeyToBuilds < ActiveRecord::Migration
  include Gitlab::Database::MigrationHelpers

  DOWNTIME = false

  disable_ddl_transaction!

  def up
    unless index_exists?(:ci_builds, :stage_id)
      add_concurrent_index(:ci_builds, :stage_id)
    end

    unless foreign_key_exists?(:ci_builds, :stage_id)
      add_concurrent_foreign_key(:ci_builds, :ci_stages, column: :stage_id, on_delete: :cascade)
    end
  end

  def down
    if foreign_key_exists?(:ci_builds, :stage_id)
      remove_foreign_key(:ci_builds, column: :stage_id)
    end

    if index_exists?(:ci_builds, :stage_id)
      remove_concurrent_index(:ci_builds, :stage_id)
    end
  end

  private

  def foreign_key_exists?(table, column)
    foreign_keys(:ci_builds).any? do |key|
      key.options[:column] == column.to_s
    end
  end
end
```
assistant