how can I register a post_meta field in an existing CPT and then call it again with get_post_custom()?

Question

I would like to add a post_meta (test_meta_1234) field to an existing CPT (organizer) of an external plugin.

With register_meta() it doesn't work. But a taxonomy I can add to the same CPT with register_taxonomy().

Code Sample:

register_meta('post', 'test_meta_1234', array(
    'object_subtype' => 'organizer',
    'show_in_rest' => true,
    'single' => true,
    'type' => 'string',
    'description' => 'Test Meta 1234',
  )
);

register_taxonomy(
'genre',
'organizer',
 array(
    'label' => __( 'Genre' ),
    'rewrite' => array( 'slug' => 'genre' ),
    'hierarchical' => true,
 )
);

$otherPostTypesFull = new stdClass();
$otherPostTypes     = get_post_types();
foreach($otherPostTypes as $postType => $postTypeSlug){
 $args = array(
              'post_type'         => $postTypeSlug,
              'numberposts'             => -1
            );
 foreach(get_posts( $args ) as $faPosts){
  $otherPostTypesFull->$postTypeSlug->post_meta   = get_post_custom($faPosts->ID);
  $otherPostTypesFull->$postTypeSlug->taxonomies  = get_object_taxonomies( $postTypeSlug, 'objects' );
 }
}
var_dump($otherPostTypesFull);

The taxonomy is added to the CPT, but not the post_meta (test_meta_1234).

Why can't I see the post_meta field with get_post_custom()?

---

update 1:

The CPT did not support 'custom-fields', so now first check and add this:

`if(!post_type_supports( 'organizer', 'custom-fields' )){
 add_post_type_support( 'organizer', 'custom-fields' );
}`

The custom-field 'test_meta_1234' is still not registered yet. Why that?

Answer 1

custom fields (post_meta) cannot be registered.

register_meta only works in conjunction with the REST API.

Answer 2

It'll probably be way easier to just use https://wordpress.org/plugins/advanced-custom-fields/

If not check out this post which points you to this page