update post meta using user_register hook not working

Question

I want to update post meta when the user registers based on the Ultimate Member plugin field in the register form. Here is the code which is working fine if I run directly on the template (loading page) but not working with the hook.

Note: As I mention the code is working as expected if I run on template file (refreshing page) but not with the hook.

function gs_add_user_to_group($user_id)
{
    // get the user group by profile
    $groupItem = get_post(um_user('team'));

    // check if post is exists and is a group post type
    if ($groupItem && $groupItem->post_type == Group_Shop_Admin::get_cpt_group()) {

        // get group post meta
        $meta         = gs_get_group_meta_key('users');
        $old_user_ids = get_post_meta($groupItem->ID, $meta, TRUE);

        // check if empty
        if ( ! $old_user_ids) {
            $user_ids = [];
        } else {
            $user_ids = $old_user_ids;
        }

        // push newly register user id
        array_push($user_ids, $user_id);

        if ( ! $old_user_ids) {
            // add new record if not exits
            add_post_meta($groupItem->ID, $meta, $user_ids);
        } else {
            // update and add newly register user id to record
            update_post_meta($groupItem->ID, $meta, $user_ids, $old_user_ids);
        }
    }
}

add_action('user_register', 'gs_add_user_to_group', 10, 1);
/* End of  gs_add_user_to_group hook */

Answer 1

The part that does not work in your code is not adding or updating the post meta, but getting the user meta named team which you're retrieving via um_user().

And that's because as @WebElaine pointed, by the time the user_register hook is fired, the custom field team has not yet added to the database (neither by WordPress nor UM), hence um_user( 'team' ) returns nothing/false.

But if you want to use the user_register hook to add/update the post meta, you can use the $_POST['team'] value submitted via the form. So replace this:

$groupItem = get_post(um_user('team'));

with this:

if ( $post_id = um_user( 'team' ) ) {
    $groupItem = get_post( $post_id );
// I'm just doing a basic validation, but you should try to make it better!
} elseif ( ! empty( $_POST['team'] ) ) {
    $groupItem = get_post( absint( $_POST['team'] ) );
}

Or since you're relying upon the team meta which came from user input submitted via the UM's registration form, then it might better to use um_registration_complete hook to add/update the post meta:

1. First, call um_fetch_user() before you call um_user():

   um_fetch_user( $user_id ); // set the user
   $groupItem = get_post( um_user( 'team' ) );
   

   Alternatively, you can use get_user_meta():

   $post_id = get_user_meta( $user_id, 'team', true );
   $groupItem = get_post( $post_id );
   

2. And replace the add_action('user_register', 'gs_add_user_to_group', 10, 1); with this:

   add_action( 'um_registration_complete', 'gs_add_user_to_group', 10, 2 );
   

Tried & tested working on WordPress 5.3.2 with UM 2.1.4.