Slug / Link to Change

function philosophy_cpt_slug_fix($post_link, $id) {
	$p = get_post($id);

	if(is_object($p) && 'chapter' == get_post_type($id)) {
		$parent_post_id = get_field('parent_book');
		$parent_post = get_post($parent_post_id);

		if($parent_post){
			$post_link = str_replace('%book%',$parent_post->post_name, $post_link);
		}
	}

	return $post_link;

}
add_filter( 'post_type_link', 'philosophy_cpt_slug_fix', 1, 2);