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: 20160417064936_add_mappings.rb
    path of file: ./repos/kitsu-server/db/migrate
    
    the code of the file until where you have to start completion: class AddMappings < ActiveRecord::Migration[4.2]
  def change
    create_table :mappings do |t|
      t.string :external_site, null: false
      t.string :external_id, null: false
      t.references :media, polymorphic: true, null: false

      t.index [:external_site, :external_id, :media_type, :media_id],
        unique: true, name: 'index_mappings_on_external_and_media'
    end


    ######### Import the existing external IDs
    #### Anime
    ## MyAnimeList
    execute <<-SQL.squish
      INSERT INTO mappings (external_site, external_id, media_type, media_id)
      SELECT 'myanimelist/anime' AS external_site, mal_id AS external_id,
             'Anime' AS media_type, id AS media_id
      FROM anime
      WHERE mal_id IS NOT NULL
    SQL
    remove_column :anime, :mal_id

    ## TheTVDB
    execute <<-SQL.squish
      INSERT INTO mappings (external_site, external_id, media_type, media_id)
      SELECT 'thetvdb/series' AS external_site, thetvdb_series_id AS external_id,
             'Anime' AS media_type, id AS media_id
      FROM anime
      WHERE thetvdb_series_id IS NOT NULL
    SQL
    remove_column :anime, :thetvdb_series_id
    execute <<-SQL.squish
      INSERT INTO mappings (external_site, external_id, media_type, media_id)
      SELECT 'thetvdb/season' AS external_site, thetvdb_season_id AS external_id,
             'Anime' AS media_type, id AS media_id
      FROM anime
      WHERE thetvdb_season_id IS NOT NULL
    SQL
    remove_column :anime, :thetvdb_season_id

    ## AnimeNewsNetwork
    execute <<-SQL.squish
      INSERT INTO mappings (external_site, external_id, media_type, media_id)
      SELECT 'animenewsnetwork' AS external_site, ann_id AS external_id,
             'Anime' AS media_type, id AS media_id
      FROM anime
      WHERE ann_id IS NOT
    
  def change
    create_table :mappings do |t|
      t.string :external_site, null: false
      t.string :external_id, null: false
      t.references :media, polymorphic: true, null: false

      t.index [:external_site, :external_id, :media_type, :media_id],
        unique: true, name: 'index_mappings_on_external_and_media'
    end


    ######### Import the existing external IDs
    #### Anime
    ## MyAnimeList
    execute <<-SQL.squish
      INSERT INTO mappings (external_site, external_id, media_type, media_id)
      SELECT 'myanimelist/anime' AS external_site, mal_id AS external_id,
             'Anime' AS media_type, id AS media_id
      FROM anime
      WHERE mal_id IS NOT NULL
    SQL
    remove_column :anime, :mal_id

    ## TheTVDB
    execute <<-SQL.squish
      INSERT INTO mappings (external_site, external_id, media_type, media_id)
      SELECT 'thetvdb/series' AS external_site, thetvdb_series_id AS external_id,
             'Anime' AS media_type, id AS media_id
      FROM anime
      WHERE thetvdb_series_id IS NOT NULL
    SQL
    remove_column :anime, :thetvdb_series_id
    execute <<-SQL.squish
      INSERT INTO mappings (external_site, external_id, media_type, media_id)
      SELECT 'thetvdb/season' AS external_site, thetvdb_season_id AS external_id,
             'Anime' AS media_type, id AS media_id
      FROM anime
      WHERE thetvdb_season_id IS NOT NULL
    SQL
    remove_column :anime, :thetvdb_season_id

    ## AnimeNewsNetwork
    execute <<-SQL.squish
      INSERT INTO mappings (external_site, external_id, media_type, media_id)
      SELECT 'animenewsnetwork' AS external_site, ann_id AS external_id,
             'Anime' AS media_type, id AS media_id
      FROM anime
      WHERE ann_id IS NOT NULL
    SQL
    remove_column :anime, :ann_id

    #### Manga
    ## MyAnimeList
    execute <<-SQL.squish
      INSERT INTO mappings (external_site, external_id, media_type, media_id)
      SELECT 'myanimelist/manga' AS external_site, mal_id AS external_id,
             'Manga' AS media_type, id AS media_id
      FROM manga
      WHERE mal_id IS NOT NULL
    SQL
    remove_column :manga, :mal_id

    #### Drama
    ## MyAnimeList
    execute <<-SQL.squish
      INSERT INTO mappings (external_site, external_id, media_type, media_id)
      SELECT 'mydramalist' AS external_site, mdl_id AS external_id,
             'Drama' AS media_type, id AS media_id
      FROM dramas
      WHERE mdl_id IS NOT NULL
    SQL
    remove_column :dramas, :mdl_id
  end