我没有使用save_postdate函数，为什么，它不保存值和更新数据

function wporg_add_custom_box(){
    $screens = ['product','wporg_cpt'];
    foreach ($screens as $screen) {
        add_meta_box(
            'wporg_box_id',// Unique ID
            'Select a Authors Name ',// Box title
            'wporg_custom_box_html',// Content callback,must be of type callable
            $screen                   // Post type
        );
    }
}
add_action('edit_form_after_title','wporg_add_custom_box');

function wporg_custom_box_html($post)
{
    $value = get_post_meta($post->ID,'_getwriter_id',true);
    ?>
    <select name="book_writer_id" id="book_writer_id" class="postbox">
        <?php
$type = 'book_authors'; // your post type
$args=array(
  'post_type' => $type,'post_status' => 'publish','posts_per_page' => -1,'caller_get_posts'=> 1
);
$my_query = null;
$my_query = new WP_Query($args);
if( $my_query->have_posts() ) {
  while ($my_query->have_posts()) : $my_query->the_post();  $id = get_the_ID();?>
     <option value="<?=$id; ?>" <?php selected($value,$id); ?>><?=the_title(); ?></option>
    <?php
  endwhile;
}
wp_reset_query();  // Restore global post data stomped by the_post().
?>
    </select>
    <?php
}

function wporg_save_postdata($post_id)
{
    if (array_key_exists('book_writer_id',$_POST)) {
        update_post_meta(
            $post_id,$_POST['book_writer_id']
        );
    }
}
add_action('save_post','wporg_save_postdata');