Reading uploaded image: undefined method `file' for nil:NilClass

问题

I receive the following error when I try to get image during thumbnail creation:

undefined method `file' for nil:NilClass

My code:

  version :thumb do
      img = ::Magick::Image::read(@file.file).first
      w = img.columns
      h = img.rows

      if w < h
        process :resize_to_limit => [30, 50]
      else
        process :resize_to_limit => [50, 30]
      end
  end

Any ideas what is this not working?

来源：https://stackoverflow.com/questions/24363366/reading-uploaded-image-undefined-method-file-for-nilnilclass