folders_controller.rb

class FoldersController < ApplicationController
  include Streamable

  def download
    @folder = Folder.find(params[:id]) # has_many_attached :files
    response.headers['Last-Modified'] = @folder.updated_at.httpdate
    zip = ZipStream.open

    send_stream(filename: "folder_#{@folder.id}.zip", type: :zip) do |stream|
      @folder.files.includes(:blob).each do |file|
        data = zip.add_stored(file.filename.to_s, file.download)
        stream.write(data)
      end

      stream.write(zip.close)
    end
  end
end

streamable.rb

module Streamable
  # Adds `send_stream` method from rails 7 alpha
  # https://github.com/rails/rails/pull/41488

  extend ActiveSupport::Concern
  include ActionController::Live

  def send_stream(filename:, disposition: 'attachment', type: nil)
    response.headers['Content-Type'] =
      (type.is_a?(Symbol) ? Mime[type].to_s : type) ||
      Mime::Type.lookup_by_extension(File.extname(filename).downcase.delete('.')) ||
      'application/octet-stream'

    response.headers['Content-Disposition'] =
      ActionDispatch::Http::ContentDisposition.format(disposition: disposition, filename: filename)

    yield response.stream
  ensure
    response.stream.close
  end
end

zip_stream.rb

require 'zip'

class ZipStream < Zip::OutputStream
  # Rolling buffer to trick RubyZip into thinking it's working with the entire zip file
  # while only holding a single entry in memory at a time
  class ZipBuffer < StringIO
    # buffer_pos keeps track of the buffer's absolute position in the zip file
    attr_accessor :buffer_pos

    def initialize(*)
      @buffer_pos = 0
      super
    end

    # RubyZip uses `tell` to calculate header offsets
    def tell
      @buffer_pos + super
    end

    # Advance buffer_pos to end of file, return contents, and clear buffer
    def dump
      @buffer_pos += size
      tmp = string.dup
      self.string = ''
      tmp
    end
  end

  class << self
    def open
      io = ZipBuffer.new('')
      io.binmode
      new(io, true)
    end
  end

  # Equivalent to Zip::File#add_stored
  def add_stored(filename, data)
    put_next_entry(filename, nil, nil, Zip::Entry::STORED)
    write(data)

    # Save current entry before finalize clears it
    entry = @current_entry
    finalize_current_entry

    # Rewrite entry updated by finalize_current_entry
    @output_stream.pos = entry.local_header_offset - @output_stream.buffer_pos
    entry.write_local_entry(@output_stream, true)

    # Return entry data and advance buffer
    @output_stream.dump
  end

  # Close zip and return final data
  def close
    return if @closed

    write_central_directory
    data = @output_stream.dump
    @output_stream.close
    @closed = true

    data
  end
end