Creating a custom ActiveStorage controller to generate "pretty urls" or help migration from another lib

custom_active_storage_urls.rb

# This demonstrates how we create a SEO friendly url for the previews of the artworks we sell.
#
# This is the URL we want:
# https://festalab.com.br/image/invitation/birthday/carnival.jpg
#

# First, the route. 
#
# "model" is the name of the active storage model that has the preview.
# "classification" and "identifier" are together a unique key for the records.
Rails.application.routes.draw do
  get "image/:model/:classification/:identifier", to: "imager/previews#show", as: :imager_preview
  
  # model         : invitation
  # classification: birthday
  # identifier    : carnival
end  

# Here's the custom controller. This is based on Rails 7.0, so you might have to change
# which controller you inherit from in previous Rails versions.
#
class Imager::PreviewsController < ActiveStorage::BaseController
  include Imager::SetBlob
  include Imager::SetVariationKey

  def show
    http_cache_forever public: true do
      send_blob_stream representation.image
    end
  rescue => error
    logger.error(error) if Rails.env.production?
    head :unprocessable_entity
  end

  private
    def representation
      @representation ||= @blob.representation(@variation_key).processed
    end
end

# This is the SetBlob concern. Instead of using the default signed_id param that active storage uses, we use our
# own parameters to locate the blob.
module Imager::SetBlob #:nodoc:
  extend ActiveSupport::Concern
  
  included do
    before_action :set_blob
  end
  
  private
    def set_blob
      model          = params[:model].constantize
      classification = params[:classification]
      identifier     = params[:identifier]
  
      @blob = model.find_by(classification: classification, identifier: identifier).preview.blob
    rescue => error
      logger.error(error) if Rails.env.production?
      head :not_found
    end
end

# This is the SetVariation concern. Instead of using the default variation_key param, we define our own transformations
# frozen_string_literal: true
module Imager::SetVariationKey #:nodoc:
  extend ActiveSupport::Concern

  included do
    before_action :set_variation_key
  end

  private
    def set_variation_key
      @variation_key = ActiveStorage::Variation.new(transformations)
    rescue => error
      logger.error(error) if Rails.env.production?
      head :not_acceptable
    end

    def transformations
      { format: :jpg, saver: {strip: true, quality: 80 }}
    end
end

Thanks!

I've chosen to include ActiveStorage::Streaming instead of extending ActiveStorage::BaseController

class PreviewController
  include ActiveStorage::Streaming
  
  def show
    http_cache_forever(public: true) do
      send_blob_stream @model
    end
  end
end