In wordpress, how do I make link redirect to login if needed, and back to the same page afterwards

I’m using the category template to list a series of custom posts, each of which lists a link to document. Some documents are public and some are private. Currently, if the post is public, the link is displayed; but if the link is private, I’m echoing a statement “Private resource — please log in to read.”

I want to make the words “log in” a link that sends any non-logged-in user to the login page and then redirects the user back to this page. But I haven’t been able to make it work. I thought maybe auth_redirect() is the way to go, but maybe I’m off base on that.

Here’s a somewhat simplified version of the loop as it currently stands.

if ( have_posts() ) :
  while ( have_posts() ) : the_post();
  // the loop
  $private = get_post_custom_values('private'); // read custom field

  if (isset($private[0]) && $private[0] == 'yes' ) { // if the post is private

    if ( is_user_logged_in() ) {// and if the user is logged in
      // display private post ?>
        <div>
          <h4><?php the_title(); ?></h4>
          <?php get_post_custom_keys();?>
          <p><?php the_content(); ?></p>
          <p><a href="<?php echo get_post_meta($post->ID, "resourcelink", true); ?>">Resource Link</a></p>
          <?php if (get_post_meta($post->ID, "private", true)==yes) {?>
            <p class="private">This document private.</p>
          <?php } ?>
        </div>
    <?php }

    else { // if the user is not logged in
      // want make the a login link, with redirect back to this page
      // but now just tells user to log in ?>
        <div>
          echo the_title('<h4>','</h4>');
          get_post_custom_keys();?>
          <p><?php the_content(); ?></p>
      <?php echo '<p>Private resource &mdash; please log in to read.</p>';
    }
  }
   else { // if the post is public
    // display public post, for every visitor ?>
      <div>
        <h4><?php the_title(); ?></h4>
        <?php get_post_custom_keys();?>
        <p><?php the_content(); ?></p>
        <p><a href="<?php echo get_post_meta($post->ID, "resourcelink", true); ?>">Resource Link</a></p>
      </div>
    <?php }
  endwhile;
endif;
?>