How to: Create random and unique filenames for all versioned files

Both of the methods below are available from Ruby 1.8.7 onwards.

Unique filenames

The following will generate UUID filenames in the following format:

1df094eb-c2b1-4689-90dd-790046d38025.jpg

someversion_1df094eb-c2b1-4689-90dd-790046d38025.jpg

class PhotoUploader < CarrierWave::Uploader::Base
  def filename
     @name ||= "#{secure_token}.#{file.extension}" if original_filename.present?
  end

  protected
  def secure_token
    var = :"@#{mounted_as}_secure_token"
    model.instance_variable_get(var) or model.instance_variable_set(var, SecureRandom.uuid)
  end
end

Note: if you do recreate_versions! this method will encode the filename of the previously encoded name, which will result in a new name

Random filenames

The following will generate hexadecimal filenames in the following format:

43527f5b0d.jpg

someversion_43527f5b0d.jpg

The length of the random filename is determined by the paramater to secure_token() within the filename method. The shorter the filename, the more chance of duplicates occurring. Unless you have a specific need for shorter filenames, it is recommended to use unique filenames instead (see above).

class PhotoUploader < CarrierWave::Uploader::Base
  def filename
     "#{secure_token(10)}.#{file.extension}" if original_filename.present?
  end

  protected
  def secure_token(length=16)
    var = :"@#{mounted_as}_secure_token"
    model.instance_variable_get(var) or model.instance_variable_set(var, SecureRandom.hex(length/2))
  end
end

(Related: How to: Use a timestamp in file names)