Build WordPress post_title from post related metadata

wp_custom_post_title_filter.php

<?php
/**
 * Assign name as title before save when post type is company_personnel.
 * 
 * Instruction of use: 
 * ------------------
 * you can place this eg. in your themes functions.php
 * 
 * Use case:
 * --------
 * You could use this with custom post type,
 * where you only want to set title programmatically. 
 * Not via the Graphical User Interface.
 *
 * @author Toni Lehtimäki
 * 
 * @param array $data
 * @param array $postarr
 *
 * @return array $_data array of post values to be saved to database
 */
function my_personnel_set_name_as_title( $data, $postarr ) {
    $_data = $data;

    // Change title if post type is company_personnel.
    if ( $_data['post_type'] === 'company_personnel' ) { 

        // Prepare for renaming the post title
        $_post_meta = get_post_meta($postarr['ID']); // Get the post metadata.
        $fname = esc_attr($_post_meta['company_person_firstname'][0]); // Get the first name from meta.
        $lname = esc_attr($_post_meta['company_person_lastname'][0]); // Get the last name meta.
        $new_title = $lname . ', ' . $fname;

        // Prepare for changing the post slug
        $post_slug = sanitize_title_with_dashes ($new_title,'','save');
        $post_slug_sanitized = sanitize_title($post_slug);

        // Set the post title only when the existing one is not equal.
        if ( $postarr['post_title'] !== $new_title ) {
	        $_data['post_title'] = $new_title;
	        $_data['post_name']  = $post_slug_sanitized;
        }

    }

    return $_data;
}
add_filter( 'wp_insert_post_data', 'my_personnel_set_name_as_title', 10, 2 );