WordPress: Add additional body classes based on the post meta

wordpress-body-classes.php

// extra body classes based on the post meta
add_filter('body_class','theme_body_class');
function theme_body_class($classes) {

	$body_classes = get_post_meta(get_the_id(), 'body-class');

	if (false !== $body_classes && count($body_classes) > 0)
		$classes = array_merge($classes, $body_classes);

	return $classes;
}