Loop through all, zip and download from ActiveStorage

downloads_controller.rb

# In our gemfile:
# gem 'rubyzip'

require 'zip'

# private ?
def process_and_create_zip_file
  # Simulation of an object with has_many_attached :documents
  job = Job.first.documents
  # Tmp folder to store the download files from S3
  # Notice we prfix the folder with a unique number (current_user.id)
  tmp_user_folder = "tmp/archive_#{current_user.id}"
  # Determin the length of the folder
  directory_length_same_as_documents = Dir["#{tmp_user_folder}/*"].length == job.documents.length
  # Create a tmp folder if not exists
  FileUtils.mkdir_p(tmp_user_folder) unless Dir.exists?(tmp_user_folder)
  # Download and save documents to our tmp folder
  job.documents.each do |document|
    filename = document.blob.filename.to_s
    # User should be able to download files if not yet removed from tmp folder
    # if the folder is already there, we'd get an error
    create_tmp_folder_and_store_documents(document, tmp_user_folder, filename) unless directory_length_same_as_documents
    #---------- Convert to .zip --------------------------------------- #
    create_zip_from_tmp_folder(tmp_user_folder, filename) unless directory_length_same_as_documents
  end
  # Sends the *.zip file to be download to the client's browser
  send_file(Rails.root.join("#{tmp_user_folder}.zip"), :type => 'application/zip', :filename => "Files_for_#{job.title.downcase.gsub(/\s+/, '_')}.zip", :disposition => 'attachment')
  # TODO: Remove files at a later date
  # as zip file wont be able to downloads if uncommented
  # FileUtils.rm_rf([tmp_user_folder, "#{tmp_user_folder}.zip"])
end

def create_tmp_folder_and_store_documents(document, tmp_user_folder, filename)
  File.open(File.join(tmp_user_folder, filename), 'wb') do |file|
    # As per georgeclaghorn in comment ;)
   document.download { |chunk| file.write(chunk) }
  end
end

def create_zip_from_tmp_folder(tmp_user_folder, filename)
  Zip::File.open("#{tmp_user_folder}.zip", Zip::File::CREATE) do |zf|
    zf.add(filename, "#{tmp_user_folder}/#{filename}")
  end
end

Nice one @SylarRuby