Redirect WordPress page to URL specified by a custom field with key "url_to_go"

redirect.php

function lets_redirect( $query ) {
	if ( !is_admin() ) {
		if ( $query->is_page() && $query->is_main_query() ) {
			$redirection = get_post_meta( $query->queried_object->ID, 'url_to_go', true );
			if ( $redirection != '' ) {
?>
				<script language="javascript" type="text/javascript">
					window.location.href="<?php echo $redirection ?>";
				</script>
<?php
			}
		}
	}
}

add_action( 'pre_get_posts', 'lets_redirect' );