WordPress：カスタム投稿でタクソノミー選択部分にあらかじめチェックを入れておく方法＋α

add_action( 'admin_head-post-new.php', 'default_taxonomy_select' );
function default_taxonomy_select() {
?>
  <script type="text/javascript">
    jQuery(function($) {
      $('#post_type_name-categorychecklist li:first-child input[type="checkbox"]').prop('checked', true);
    });
  </script>
<?php
}

function add_default_term_setting_item() {
  $post_types = get_post_types( array( 'public' => true, 'show_ui' => true ), false );
  if ( $post_types ) {
    foreach ( $post_types as $post_type_slug => $post_type ) {
      $post_type_taxonomies = get_object_taxonomies( $post_type_slug, false );
      if ( $post_type_taxonomies ) {
        foreach ( $post_type_taxonomies as $tax_slug => $taxonomy ) {
          if ( ! ( $post_type_slug == 'post' && $tax_slug == 'category' ) && $taxonomy->show_ui ) {
            add_settings_field( $post_type_slug . '_default_' . $tax_slug, $post_type->label . '用' . $taxonomy->label . 'の初期設定' , 'default_term_setting_field', 'writing', 'default', array( 'post_type' => $post_type_slug, 'taxonomy' => $taxonomy ) );
          }
        }
      }
    }
  }
}
add_action( 'load-options-writing.php', 'add_default_term_setting_item' );

function default_term_setting_field( $args ) {
  $option_name = $args['post_type'] . '_default_' . $args['taxonomy']->name;
  $default_term = get_option( $option_name );
  $terms = get_terms( $args['taxonomy']->name, 'hide_empty=0' );
  if ( $terms ) :
?>
<select name="<?php echo $option_name; ?>">
    <option value="0">設定しない</option>
<?php foreach ( $terms as $term ) : ?>
    <option value="<?php echo esc_attr( $term->term_id ); ?>"<?php echo $term->term_id == $default_term ? ' selected="selected"' : ''; ?>><?php echo esc_html( $term->name ); ?></option>
<?php endforeach; ?>
  </select>
<?php
  else:
?>
  <p><?php echo esc_html( $args['taxonomy']->label ); ?>が登録されていません。</p>
<?php
  endif;
}

function allow_default_term_setting( $whitelist_options ) {
  $post_types = get_post_types( array( 'public' => true, 'show_ui' => true ), false );
  if ( $post_types ) {
    foreach ( $post_types as $post_type_slug => $post_type ) {
      $post_type_taxonomies = get_object_taxonomies( $post_type_slug, false );
      if ( $post_type_taxonomies ) {
        foreach ( $post_type_taxonomies as $tax_slug => $taxonomy ) {
          if ( ! ( $post_type_slug == 'post' && $tax_slug == 'category' ) && $taxonomy->show_ui ) {
            $whitelist_options['writing'][] = $post_type_slug . '_default_' . $tax_slug;
          }
        }
      }
    }
  }
  return $whitelist_options;
}
add_filter( 'whitelist_options', 'allow_default_term_setting' );

function add_post_type_default_term( $post_id, $post ) {
  if ( ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE ) || $post->post_status == 'auto-draft' ) { return; }
  $taxonomies = get_object_taxonomies( $post, false );
  if ( $taxonomies ) {
    foreach ( $taxonomies as $tax_slug => $taxonomy ) {
      $default = get_option( $post->post_type . '_default_' . $tax_slug );
      if ( ! ( $post->post_type == 'post' && $tax_slug == 'category' ) && $taxonomy->show_ui && $default && ! ( $terms = get_the_terms( $post_id, $tax_slug ) ) ) {
        if ( $taxonomy->hierarchical ) {
          $term = get_term( $default, $tax_slug );
          if ( $term ) {
            wp_set_post_terms( $post_id, array_filter( array( $default ) ), $tax_slug );
          }
        } else {
          $term = get_term( $default, $tax_slug );
          if ( $term ) {
            wp_set_post_terms( $post_id, $term->name, $tax_slug );
          }
        }
      }
    }
  }
}
add_action( 'wp_insert_post', 'add_post_type_default_term', 10, 2 );