How to enforce a requirement for custom taxonomy

Question

I’d like to set a custom taxonomy as required, i.e. authors must select at least one term from the taxonomy in order to save or update a post of a specified set of custom post type. If they haven't done that, a custom error message displays.

I’m registering taxonomies using Extended CPTs, for example:

<?php
// Topic taxonomy
// Docs: https://github.com/johnbillion/extended-cpts/wiki/Registering-taxonomies
add_action('init', function () {
  register_extended_taxonomy('topic', [
    'article',
    'event',
    'resource',
  ], [
    'exclusive' => false,
    'hierarchical' => false,
    'meta_box' => 'simple', // supports multiple (non-exclusive)
    'public' => true, // applies to “publicly_queryable”, “show_ui”, and “show_in_nav_menus”
    'query_var' => true,
    'required' => true,
    'show_admin_column' => true,
    'show_in_quick_edit' => true,
    'show_in_rest' => true,
  ], [
    'singular' => 'Topic *',
    'plural' => 'Topics',
    'slug' => 'topic',
  ]);
});

How can this be accomplished (ideally, without using jQuery)?

Answer 1

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.