Base Uploader

Let’s start at the bottom with the Uploadable module. Generate it as a CarrierWave uploader with:

bin/rails g uploader uploadable

Then move it into app/uploaders/concerns (you’ll have to create this directory), rename it to uploadable.rb, and change it from a class to a module that extends ActiveSupport::Concern¹:

module Uploadable
  extend ActiveSupport::Concern

  included do
    # …process, version, &c.
  end

  # …the rest of the uploader class goes here
end

Do whatever configuration in this module that you want all of your uploaders to have or be able to override. Put any CarrierWave DSL method calls (e.g. process) in the included block. All of this module’s instance methods will be added to the instances of any uploader class that includes it.

A Basic Uploader

The simplest implementation of an uploader that inherits our new base uploader looks like this:

class SimpleUploader < CarrierWave::Uploaders::Base
  include Uploadable
end

This uploader uses all the defaults set in Uploadable, and can be mounted and used like any other CarrierWave uploader. You can add or override any methods (e.g. store_dir) or versions defined in the base uploader, and they will remain specific to files uploaded with this uploader.

The Second Level

The one catch with this is that versions and processors defined in the base uploader will be used for every uploader that includes it. Sometimes this isn’t what you want: you may want some kinds of image uploads to have a thumbnail, and others to be converted to JPG on upload, and want multiple uploaders to use one or the other or both sets of configuration. Easy; just make more concern modules:

module Jpegable
  extend ActiveSupport::Concern
  include Uploadable
  include CarrierWave::MiniMagick

  included do
    process convert: 'jpg'
  end
end

module Thumbnailable
  extend ActiveSupport::Concern
  include Uploadable
  include CarrierWave::MiniMagick

  included do
    version :thumb do
      process resize_to_fit: [100, 100]
    end
  end
end

Since each of these concerns also includes Uploadable, there’s no need to include it in uploaders that use either or both of them²:

class ImageUploader < CarrierWave::Uploader::Base
  include Jpegable
  include Thumbnailable
end