How to enforce a requirement for custom taxonomy

You could consider using the wp_insert_post_empty_content hook. This hook determines if a post is considered empty and if so, prevents it from being saved. For example:

/**
 * Checks whether some post types have a topic.
 *
 * @param bool  $maybe_empty Whether the post should be considered "empty".
 * @param array $postarr     Array of post data.
 * @return bool Whether the post is empty.
 */
function my_plugin_require_topic( $maybe_empty, $postarr ) {
  if ( ! $maybe_empty && in_array( $postarr['post_type'], array( 'article', 'event', 'resource' ), true ) {
    $maybe_empty = empty( $postarr['tax_input']['topic'] );
  }

  return $maybe_empty;
}
add_filter( 'wp_insert_post_empty_content', 'my_plugin_require_topic', 10, 2 );

For how to implement the custom error message, that would depend on where you want the error message surfaced. This is because there are multiple ways that a post could be edited, and surfacing the error message would depend on the interface you’d want to target.

Related Posts: