ActiveStorage migration guide and caveats from Carrierwave (similar starting point for Paperclip)

README.md

Migrate Carrierwave to ActiveStorage

Migrate, e.g. organisation.logo (Carrierwave "LogoUploader") to organisation.logo2 (ActiveStorage)

require 'migrate_attachment'
migrate_attachment!(
  klass: Organisation, 
  attachment_attribute: :logo, 
  carrierwave_uploader: LogoUploader, 
  active_storage_column: :logo2
)

Checklist:

• Use with caution! only tested with file adapters, not Cloud storage yet.
• In this example we migrate from logo to logo2 so we can replace all occurences in the codebase. If you are confident, you could also migrate to the same column name (logo -> logo). But make sure to replace all logo.url(...) stuff etc.
• Only works if you not made any funky things with Carrierwave path (e.g. base path on parts of model methods). Add the missing methods to the meta programming part, as well as a vanilla table with "normal" id column.
• Does not migrate any versions. Try to mimick your Carrierwave versions like:

  mount_uploader :logo, LogoUploader
  ...
  version :medium do
    process resize_to_fit: [200, 1500]
  end
  
# becomes:
  
  has_one_attached :logo2

  def logo_medium
    logo2.variant(
      combine_options: {
        gravity: "center",
        resize: "200x1500>",
        crop: "200x1500+0+0"
      })
  end

• programatically setting blobs + nil does not work. Use:
  • organisation.logo2.attach(io: tempfile, filename: filename), instead of organisation.logo = tempfile
  • organisation.logo2.detach instead of organisation.logo2 = nil
• referencing:
  • image_tag organisation.logo2
  • if need url e.g. for serializer or inside pdf scripts etc, one need to use the Rails url helper:
    • blob: Rails.application.routes.url_helpers.rails_blob_path(organisation.logo2, only_path: true)
    • variant: Rails.application.routes.url_helpers.rails_representation_path(organisation.logo_medium, only_path: true)
• AS does not have a remote_ATTACHMENT that downloads automatically from remote server
• grep the codebase for organisation.logo.url and organisation.logo.path, organisation.logo.versions
• If you need to access the blob for local processing, use AS::Downloader config in accompanied file (organisation.logo2.open { |tempfile| .... }), if on Rails 5.2.0.

migrate_attachment.rb

# lib/migrate_attachment.rb
def migrate_attachment!(klass:, attachment_attribute:, carrierwave_uploader:, active_storage_column: attachment_attribute)
  old_class_name = "Old#{klass.name}"
  new_class_name = "New#{klass.name}"

  eval <<~DOC
    class #{new_class_name} < ActiveRecord::Base
      self.table_name = #{klass.name}.table_name
      has_one_attached :#{active_storage_column}
    end

    class #{old_class_name} < ActiveRecord::Base
      self.table_name = #{klass.name}.table_name
      mount_uploader :#{attachment_attribute}, #{carrierwave_uploader.name}

      def self.to_s
        #{klass}.to_s
      end
    end

  DOC

  new_class = new_class_name.constantize

  old_class_name.constantize.find_each do |item|
    next unless item.send(attachment_attribute).present?

    attachment = item.send(attachment_attribute)
    attachment.cache_stored_file!
    file = attachment.sanitized_file.file
    content_type = item.send(attachment_attribute).content_type
    copy = new_class.find(item.id)
    copy.send(active_storage_column).attach(io: file, content_type: content_type, filename: item.attributes[attachment_attribute.to_s])
    copy.save
  end

  ActiveStorage::Attachment.where(record_type: new_class_name).update_all record_type: klass.to_s
end

zz_as_downloader.rb

# config/initializers/activestorage.rb
# frozen_string_literal: true

if Rails.version > "5.2.0"
  raise "Check if this commit is already included in your current rails version:

  https://github.com/rails/rails/commit/ee21b7c2eb64def8f00887a9fafbd77b85f464f1#diff-3fd88ddd945ad24c4bd6f76c64c8790a
  "
end

module ActiveStorage
  class Downloader #:nodoc:
    def initialize(blob, tempdir: nil)
      @blob    = blob
      @tempdir = tempdir
    end

    def download_blob_to_tempfile
      open_tempfile do |file|
        download_blob_to file
        verify_integrity_of file
        yield file
      end
    end

    private
      attr_reader :blob, :tempdir

      def open_tempfile
        file = Tempfile.open([ "ActiveStorage-#{blob.id}-", blob.filename.extension_with_delimiter ], tempdir)

        begin
          yield file
        ensure
          file.close!
        end
      end

      def download_blob_to(file)
        file.binmode
        blob.download { |chunk| file.write(chunk) }
        file.flush
        file.rewind
      end

      def verify_integrity_of(file)
        unless Digest::MD5.file(file).base64digest == blob.checksum
          raise ActiveStorage::IntegrityError
        end
      end
  end
end

module AsDownloadPatch
  def open(tempdir: nil, &block)
    ActiveStorage::Downloader.new(self, tempdir: tempdir).download_blob_to_tempfile(&block)
  end
end

Rails.application.config.to_prepare do
  ActiveStorage::Blob.send(:include, AsDownloadPatch)
end

Use with caution! only tested with file adapters, not Cloud storage yet.

Has it been tested with AWS at this point?