ActiveStorage Downloader takes attachment input and yields a tempfile

downloader.rb

# frozen_string_literal: true

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

somewhere_in_your_code.rb

ActiveStorage::Downloader.new(attachment).download_blob_to_tempfile do |tempfile|
  do_stuff_with_file(tempfile)
end