How to Rewrite Guest Author Name with Custom Fields in WordP

add_filter( 'the_author', 'guest_author_name' );
add_filter( 'get_the_author_display_name', 'guest_author_name' );
function guest_author_name( $name ) {
global $post;
$author = get_post_meta( $post->ID, 'author', true );
if ( $author )
$name = $author;
return $name;
}