CarrierWave Process Parameters (or: MakeIt800.com, Casualty of the Zeitgeist)

I have an idea for a new app, MakeIt800.com! Users can upload any photo and receive a copy scaled to an 800-pixel square, for optimum convenience.

I want to build my fledgling empire on Rails, and upload my photos with CarrierWave:

class PhotoUploader < CarrierWave::Uploader::Base
  process resize_to_fill: [800, 800]
end

Awesome, app done, legacy secured.

Later that afternoon, one of the interns interrupts my sun-bathing to brag about her weekend project, square.it — an app that processes photos into squares of any size, with futuristic URLs like square.it/800, or even square.it/900!

After brief consideration of legal action, I resolve to overcome my opponent with sheer superiority. But how has she accomplished this? CarrierWave doesn’t seem to allow me to pass parameters from a route into an uploader’s processes!

The intern I wrangled for this work had some notes:

# Storing the parameters on the uploader isn’t great, but
# it’s better than putting them on the model
class PhotoUploader < CarrierWave::Uploader::Base
  attr_accessor :size
  process resize_to_fill: [@size, @size]
end

# Note that we need to set our new uploader values AFTER
# the model is initialized, but BEFORE we trigger
# CarrierWave’s processing
class ConversionsController < ApplicationController
  def create
    conversion = Conversion.new
    conversion.photo.size = params[:size] # ← Right here!
    conversion.photo = params[:photo]
    # ...
  end
end

# For a custom process (maybe with many parameters), we can
# move the processing and parameters into a separate module
class PhotoUploader < CarrierWave::Uploader::Base
  include AlterImage
end
module AlterImage
  extend ActiveSupport::Concern
  attr_accessor :a, :b, :c, :d
  included do
    process :my_custom_alteration
  end
  private
  def my_custom_alteration
    # ...
  end
end

# Testing fixtures or factories that rely on our process
# will also need to keep these operations in the right order
FactoryGirl.define do
  factory :conversion do
    before(:create) do |instance|
      instance.photo.size = 800
      instance.photo = photo_fixture
    end
    # ...
  end
end

Now — I’m not sure what any of that means, but I’ve been assured that make.it/800 debuts tomorrow!