get_posts works but new wp_query doesn't

Question

I was told that get_posts is a very bad function to use that that making a new WP_query is absolutely better.

My problem is that my get_posts code works but when I tried turning it into a WP_query, it returns blank results. Assistance please? :( I'd really love to get this to work because I suspect that the get_posts that I have on index.php / category-9.php are interfering with the normal loop on category.php

Get_posts version:

<?php if ( have_posts() ) : while ( (have_posts() ) ) : the_post(); ?>
  <div class="postcontainer">
    <h2><a href="<?php the_permalink();?>"><?php the_title(''); ?></a></h2>

    <?php
    $args = array(
        'post_type' => 'attachment',
        'post_mime_type' => 'image',
        'numberposts' => 4,
        'post_status' => null,
        'post_parent' => $post->ID,
        'orderby' => name,
        'order' => ASC
    );
    $attachments = get_posts($args);
    if ($attachments) {
      foreach ($attachments as $post) {
        setup_postdata($post); ?>
          <a class="postpreview" href="<?php echo get_permalink($post->post_parent) ?>"><?php echo wp_get_attachment_image($post->ID, thumbnail) ?></a>
      <?php }; 
    }; ?>
    <div class="clear"></div>
  </div>
<?php endwhile; endif; wp_reset_query(); ?>

WP_query version:

<?php if ( have_posts() ) : while ( (have_posts() ) ) : the_post(); ?>
  <div class="postcontainer">
    <h2><a href="<?php the_permalink();?>"><?php the_title(''); ?></a></h2>

    <?php
    $args = array(
        'post_type' => 'attachment',
        'post_mime_type' => 'image',
        'numberposts' => 4,
        'post_status' => null,
        'post_parent' => $post->ID,
        'orderby' => name,
        'order' => ASC
    );
    $attachments = new WP_Query($args);
      while ($attachments->have_posts()) : ?>
          <a class="postpreview" href="<?php echo get_permalink($post->post_parent) ?>"><?php echo wp_get_attachment_image($post->ID, thumbnail) ?></a>
      <?php endwhile; wp_reset_postdata(); ?>
    <div class="clear"></div>
  </div>
<?php endwhile; endif; ?>

Answer 1

You forget $query->the_post(), which makes the same as setup_postdata(). Also you are using $post->ID inside the loop but $post is not current post object in the loop. You have also syntax errors; for example you pass thumnail to wp_get_attachment_image() instead of a string "thumbnail", and pass name as orderby argument instead of the string "name", or ASC instead of "ASC":

 <?php
 //as you are inside a loop but I don't see $post variable in your code,
 //I assume you want the current post in the loop as parent
 $parent_id = get_the_ID();

 $args = array(
    'post_type' => 'attachment',
    'post_mime_type' => 'image',
    'posts_per_page' => 4,
    'post_status' => 'inherit',
    'post_parent' => $parent_id,
    'orderby' => 'name',
    'order' => 'ASC'
);

$attachments = new WP_Query($args);
while ($attachments->have_posts()) {
       $attachments->the_post();
       //I pass $parent_id to get_permalink() because you was trying
       //to do that in your code but I'm not sure if this is what you want
       ?>
      <a class="postpreview" href="<?php echo get_permalink($parent_id); ?>"><?php echo wp_get_attachment_image(get_the_ID(), 'thumbnail') ?></a>

 <?php
 }
 wp_reset_postdata();
 ?>

Anyway, new WP_query is not much better than get_posts(), each one has its benefits and the better depends on what you want to do.