Hooking to walker_nav_menu_start_el to insert list of subpages

Question

What I want

If a 'category' is added to the menu using the 'menus' tool in WordPress, I want my theme to automatically add a submenu containing links to all of the posts in that category.

<ul>
    <li><a href="link">Page 1</a></li>
    <li><a href="link">Page 2</a></li>
    <li><a href="link">Category X</a>
        //Automatically insert list of links to all posts in Category X
        <ul>
            <li><a href="link">Post 1</a></li>
            <li><a href="link">Post 2</a></li>
        </ul>
    </li>
    <li><a href="link">Category Y</a>
        //Automatically insert list of links to all posts in Category Y
        <ul>
            <li><a href="link">Post 1</a></li>
            <li><a href="link">Post 2</a></li>
            <li><a href="link">Post 3</a></li>
            <li><a href="link">Post 4</a></li>
        </ul>
    </li>
</ul>

What I've tried

In my functions.php file I have created a function and hooked it to walker_nav_menu_start_el action. By passing the $item variable into the function I am able to identify if the current menu item is a category and what category it is. But I'm having trouble creating the list of links with post titles for all posts in that category.

At first I thought I could use the get_posts function and a foreach loop, but because I'm not outputting, I'm just building an output variable to be passed back to the walker_nav_menu_start_el action, it's merely outputting the details of the current post instead of the details for each post it loops through. This is the code I tried:

function hw_submenu( $output, $item ) {
   if( $item->object == 'category' ) {

      $hw_output = '<ul>';
      $subposts = get_posts( array(
         'posts_per_page' => -1,
         'category' => $item->object_id
      ) );
      if ( $subposts ) {
         foreach ( $subposts as $post ) :
            setup_postdata( $post );
            $hw_output .= "<li><a href='" . the_permalink() . "''>" . the_title('', '', false) . "</a></li>";
         endforeach; 
         wp_reset_postdata();
      }

      $output .= $hw_output . "</ul>";
   }    

   return $output;
}
add_action( 'walker_nav_menu_start_el', 'hw_submenu', 10, 2);

And this is an example menu output:

<ul>
    <li><a href="link">Page 1</a></li>
    <li><a href="link">Page 2</a></li>
    <li><a href="link">Category X</a>
        //Automatically inserts list of links to the post I'm currently viewing
        <ul>
            <li><a href="link">Post I'm currently viewing</a></li>
            <li><a href="link">Post I'm currently viewing</a></li>
        </ul>
    </li>
    <li><a href="link">Category Y</a>
        //Automatically inserts list of links to the post I'm currently viewing
        <ul>
            <li><a href="link">Post I'm currently viewing</a></li>
            <li><a href="link">Post I'm currently viewing</a></li>
            <li><a href="link">Post I'm currently viewing</a></li>
            <li><a href="link">Post I'm currently viewing</a></li>
        </ul>
    </li>
</ul>

How can I get each category to list its posts in the menu? Can I do it with this action hook? Or should I be looking at a custom walker?

Answer 1

I was able to solve my own problem by using get_the_title with the post ID rather than using the_title. Then I decided to try using $subpost->post_title and that worked too. I figure this is better than get_the_title since it doesn't have to "get" the data again, it just uses the data we've already gotten. I have no idea why $subpost->post_title would work and the_title wouldn't. Would love to hear any insights people have about this.

Working code:

function hw_submenu( $output, $item ) {
   if( $item->object == 'category' ) {

      $hw_output = '<ul>';
      $subposts = get_posts( array(
         'posts_per_page' => -1,
         'category' => $item->object_id
      ) );
      if ( $subposts ) {
         foreach ( $subposts as $subpost ) :
            setup_postdata( $subpost );
            $hw_output .= "<li><a href='" . get_the_permalink($subpost->ID) . "''>" . $subpost->post_title . "</a></li>";
         endforeach; 
         wp_reset_postdata();
      }

      $output .= $hw_output . "</ul>";
   }    

   return $output;
}
add_filter( 'walker_nav_menu_start_el', 'hw_submenu', 10, 2);