You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: 20170118222403_fill_manga_age_ratings.rb
    path of file: ./repos/kitsu-server/db/migrate
    
    the code of the file until where you have to start completion: require 'update_in_batches'

class FillMangaAgeRatings < ActiveRecord::Migration[4.2]
  using UpdateInBatches

  self.disable_ddl_transaction!

  def up
    # Sorted by severity of rati
    
  def up
    # Sorted by severity of rating
    {
      'Kids'          => [:G, 'Kids'],
      'Harem'         => [:PG],
      'Gender Bender' => [:PG],
      'Romance'       => [:PG],
      'Yuri'          => [:PG],
      'Yaoi'          => [:PG],
      'Gore'          => [:R, 'Gore'],
      'Ecchi'         => [:R, 'Ecchi'],
      'Doujinshi'     => [:R],
      'Horror'        => [:R, 'Horror'],
      'Hentai'        => [:R18, 'Hentai']
    }.each do |genre, (rating, reason)|
      update_rating_for_genre(rating, genre, reason)
    end
  end