Paperclip change path migration helper

gistfile1.rb

# WARNING: don't just use this without testing!
# This is not a database migration but i think it's quite convenient to do as a migration
# multiple styles has not been tested but should work

class ChangeAttachmentPaths < ActiveRecord::Migration
  AttachmentPath = Rails.root.join("public", "system")

  def self.paperclip_change_path(table, column, old_opt, new_opt)
    model = table.to_s.classify.constantize

    # build {[id, style] => [old_path, new_path], ...}
    rename_map = {}
    model.has_attached_file column, old_opt
    model.all.each do |r|
      c = r.send(column)
      next if not c.file?
      styles = c.styles.keys
      styles = [:original] if styles.empty?
      styles.each do |style|
        rename_map[[r.id, style]] = [c.path(style)]
      end
    end
    model.has_attached_file column, new_opt
    model.all.each do |r|
      c = r.send(column)
      styles = c.styles.keys
      styles = [:original] if styles.empty?
      styles.each do |style|
        next if not rename_map.has_key? [r.id, style]
        rename_map[[r.id, style]].push(c.path(style))
      end
    end

    # rename files
    rename_map.values.each do |old_path, new_path|
      #puts "FileUtils.mkdir_p #{File.dirname(new_path)}"
      #puts "File.rename #{old_path}, #{new_path}"
      FileUtils.mkdir_p File.dirname(new_path)
      File.rename old_path, new_path
    end

    # remove empty directories
    Dir[AttachmentPath.join(table.to_s, column.to_s.pluralize, "**/*")]
      .select { |d| File.directory? d }
      .select { |d| (Dir.entries(d) - %w[ . .. ]).empty? }
      .each { |d| Dir.rmdir d }
  end

  def self.up
    paperclip_change_path :users, :profile_image,
      {:path => ":rails_root/public:url", :url => "/system/:class/:attachment/:id/:style.:extension"},
      {:path => ":rails_root/public:url", :url => "/system/:class/:attachment/:id/:filename"}

  end

  def self.down
    paperclip_change_path :users, :profile_image,
      {:path => ":rails_root/public:url", :url => "/system/:class/:attachment/:id/:filename"},
      {:path => ":rails_root/public:url", :url => "/system/:class/:attachment/:id/:style.:extension"}
  end
end