Rails Activestorage apply variant before uploading. Crop image before uploading. Process image before uploading.

activestorage.rb

# config/initializers/activestorage.rb

# Ability to auto apply :default variant (if defined) before uploading original image
Rails.application.config.after_initialize do
  ActiveStorage::Blob.class_eval do
    alias_method :upload_without_unfurling_orig, :upload_without_unfurling
    
    def upload_without_unfurling(io)
      variant = attachments.first.send(:variants)
      default_variant = variant[:default]
      if default_variant
        ActiveStorage::Variation.wrap(default_variant).transform(io) do |output|
          unfurl output, identify: identify
          upload_without_unfurling_orig(output)
        end
      else
        upload_without_unfurling_orig(io)
      end
    end
  end
end

user.rb

class User < ApplicationRecord
  has_one_attached :photo do |attachable|
    attachable.variant :thumb, resize_to_fill: [100, nil]
    # This variant will be applied before uploading the original image
    attachable.variant :default, strip: true, quality: 70, resize_to_fill: [500, 500]
  end
end

user_spec.rb

require 'rails_helper'
RSpec.describe User, type: :model do
  describe 'when uploading photo' do
    it 'applies :default variant before uploading the original image' do
      allow(ActiveStorage::Variation).to receive(:wrap).and_call_original
      create(:user, :with_photo)
      exp_args = hash_including(resize_to_fill: [500, 500])
      expect(ActiveStorage::Variation).to have_received(:wrap).with(exp_args)
    end
  end
end