public void addAuthor(Author author) {
  try(Connection con = DB.sql2o.open()) {
    String sql = "INSERT INTO books_authors (book_id, author_id) VALUES (:book_id, :author_id)";
    con.createQuery(sql)
      .addParameter("author_id", book.getId())
      .addParameter("book_id", this.getId())
      .executeUpdate();
  }
}


public List<Author> getAuthors() {
  try(Connection con = DB.sql2o.open()){
    String sql = "SELECT authors.* FROM books JOIN books_authors ON (books.id = books_authors.book_id) JOIN authors ON (books_authors.author_id = authors.id) where books.id= :book_id;";
       List<Autho> authors = con.createQuery(sql)
      .addParameter("book_id", this.getId())
      .executeAndFetch(Author.class);
    return books;
  }
}