There are lots of cases in which, the user wants to download his uploaded documents for offline purpose.  So in rails application it is very easy to achieve this task.
I am using dragonfly gem in my application for file handling. To create a zip I uses rubyzip gem. The demo code is as follows:

Consider that you a have table named as photos and you want to send a zip for it.

#to use rubyzip gem you need to first require it in your code
require 'zip/zip'

def download_photos
photos = Photo.find(:all)

#set the zip file name. For now it is under tmp directory of your rails application
zip_name = "#{Rails.root}/tmp/#{Time.now}.zip"


#delete the existing file so that it always creates a fresh file
File.delete(zip_name) if File.exist?(zip_name)

    Zip::ZipFile.open(zip_name, Zip::ZipFile::CREATE) do |zipfile|
      photos.each do |photo|
        unless photo.cover_image_uid.blank?
          begin
            zipfile.add(photo.name, "#{Rails.root}/public/system/dragonfly
                                                      /#{Rails.env}/#{photo.cover_image_uid}")
          rescue Zip::ZipEntryExistsError => e
            logger.info "Unable to add the following screenshot #{photo.name} of
                                   test_case_revision #{tcr.id} into zip file."
          end
        end
      end
    end
    send_file zip_name

end

Use it and complete your task.
Please let me your of doubts\queries.