O que tu prefere

register_owner.rb

def register_owner
    uuid = qr_code_params[:uuid]
    qr_code = QrCode.find_by(uuid: uuid)
    if(qr_code)
      if(qr_code.user)
        render json: QrCodeAlreadyHasAnOwnerError.new(uuid), status: :conflict
      else
        @current_user.qr_codes << qr_code
        render json: qr_code, status: :created
      end
    else
      render json: QrCodeNotFoundError.new(uuid), status: :not_found
    end
  end
  
def register_owner
  uuid = qr_code_params[:uuid]
  qr_code = QrCode.find_by(uuid: uuid)
  return json: QrCodeNotFoundError.new(uuid), status: :not_found unless qr_code
  return json: QrCodeAlreadyHasAnOwnerError.new(uuid), status: :conflict unless qr_code.user

  @current_user.qr_codes << qr_code
  render json: qr_code, status: :created
end

👇