class ImageUploader < CarrierWave::Uploader::Base
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  省略
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  .
  .
  def filename
    "#{mounted_as}_#{Time.now.to_i}.#{file.extension}" if original_filename
  end
end

class ImageUploader < CarrierWave::Uploader::Base
  .
  .
  .
  省略
  .
  .
  .
  def filename
    "#{mounted_as}_#{timestamp}.#{file.extension}" if original_filename
  end

  def timestamp
    var = :"@#{mounted_as}_timestamp"
    model.instance_variable_get(var) or model.instance_variable_set(var, Time.now.to_i)
  end
end