carrierwave mount uploader 的位置问题，奇怪

  ### callbacks
  before_save :assign_qr_code

  ### mount uploader
  mount_uploader :qr_code_image, QrCodeUploader
  # 上面这两行代码换位置就不行了，为啥？
  private
  def assign_qr_code
      if self.qr_code_image.url == nil
          tmp_path = Rails.root.join('tmp', "qrcode.png")
      png = RQRCode::QRCode.new( qrcode_url, :size => 4, :level => :h ).to_img.resize(250, 250).save(tmp_path)
        File.open(tmp_path) do |file|
          self.qr_code_image = file
        end
        File.delete(tmp_path)
    end
  end
  def qrcode_url
    "http://hi.baidu.com"
  end