CarrierWave on the fly resizer (work as dragonfly)

image_uploader.rb

class ImageUploader < CarrierWave::Uploader::Base
  include CarrierWave::MiniMagick
  storage :file

  def thumb(size)
    begun_at = Time.now
    size.gsub!(/#/, '!')
    uploader = Class.new(self.class)
    uploader.versions.clear
    uploader.version_names = [size]
    img = uploader.new
    img.retrieve_from_store!(identifier)
    cached = File.join(CarrierWave.root, img.url)
    unless File.exist?(cached)
      img.cache!(self)
      img.send(:original_filename=, original_filename)
      size = size.split('x').map(&:to_i)
      resizer = case size
                when /[!#]/ then :resize_to_fit
                # add more like when />/ then ...
                else :resize_to_fill
                end
      img.send(resizer, *size)
      img.store!
      logger.debug 'RESIZE', begun_at, img.store_path
    end
    img
  end

  def store_dir
    'uploads'
  end

  def cache_dir
    Padrino.root('tmp/uploads')
  end

  def extension_white_list
    %w[jpg jpeg gif png]
  end

  def filename
    Digest::MD5.hexdigest(original_filename) << File.extname(original_filename) if original_filename
  end

  def default_url
    '/images/no-image.png'
  end
end

test.out

>> i = ImageUploader.new
=> #<ImageUploader:0x1044314f8 @model=nil, @mounted_as=nil>
>> i.download!('http://a1.twimg.com/profile_images/682283839/PadrinoLogo_reasonably_small.jpg')
=> [:process!, :assign_parent_cache_id, :cache_versions!]
>> i.store!
=> [:store_versions!]
>> i.url
=> "/uploads/35c40bbe3a9170e2901165990c34a173.jpg"
>> i.thumb('100x100').url
  DEBUG -   RESIZE (0.1124ms) uploads/100x100_35c40bbe3a9170e2901165990c34a173.jpg
=> "/uploads/100x100_35c40bbe3a9170e2901165990c34a173.jpg"
>> i.thumb('100x100').url
=> "/uploads/100x100_35c40bbe3a9170e2901165990c34a173.jpg"

Also, on line 17, you overwrite size, stripping out any !# that would trigger the different manipulation types in the case statement (regex compare) just below.

size = size.split('x').map(&:to_i)
resizer = case size
when /[!#]/ then :resize_to_fit