问在Ruby回形针GEM中获取模型中图像的宽度和高度

EN

has_attached_file :photo,
                      :styles => {
                      :original  => "634x471>",
                      :thumb => Proc.new { |instance|                       
                                  ratio = instance.photo_width/instance.photo_height
                                  min_width   = 142
                                  min_height  = 119
                                  if ratio > 1
                                    final_height  = min_height
                                    final_width   = final_height * ratio
                                  else
                                    final_width   = min_width
                                    final_height  = final_width * ratio
                                  end
                                  "#{final_width}x#{final_height}" 
                                }
                    }, 
                    :storage => :s3,
                    :s3_credentials => "#{RAILS_ROOT}/config/s3.yml",
                    :path => ":attachment/:id/:style.:extension",
                    :bucket => 'foo_bucket'