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: 20100907210915_include_note_body_in_history_fts.rb
    path of file: ./repos/moebooru/db/migrate
    
    the code of the file until where you have to start completion: class IncludeN
    
  def self.up
    # For most value fields, just index it directly.  For cached_tags, index the changes compared
    # to the previous value.
    execute """
    CREATE OR REPLACE FUNCTION history_changes_index_trigger() RETURNS trigger AS $$
    DECLARE
      old_tags varchar;
      old_tags_array varchar[];
      new_tags_array varchar[];
      changed_tags_array varchar[];
      indexed_value varchar;
    BEGIN
      IF (new.table_name, new.field) IN (('posts', 'cached_tags')) THEN
        old_tags := prev.value FROM history_changes prev WHERE (prev.id = new.previous_id) LIMIT 1;
        old_tags_array := regexp_split_to_array(COALESCE(old_tags, ''), ' ');
        new_tags_array := regexp_split_to_array(COALESCE(new.value, ''), ' ');

        changed_tags_array := get_new_tags(old_tags_array, new_tags_array);
        changed_tags_array := array_cat(changed_tags_array, get_new_tags(new_tags_array, old_tags_array));
        indexed_value := join_string(changed_tags_array, ' ');
      ELSEIF (new.table_name, new.field) IN (('pools', 'name')) THEN
        indexed_value := translate(new.value, '_', ' ');
      ELSEIF (new.table_name, new.field) IN (('posts', 'cached_tags'), ('posts', 'source'), ('pools', 'description'), ('notes', 'body')) THEN
        indexed_value := new.value;
      ELSE
        RETURN new;
      END IF;

      new.value_index := to_tsvector('public.danbooru', indexed_value);

      RETURN new;
    END
    $$ LANGUAGE plpgsql;
    """

    # We added notes since the previous update; make sure those search indexes are updated.
    execute "UPDATE history_changes SET value = value WHERE table_name = 'notes' AND field = 'body'"
  end