How to order taxonomy terms by most recent post?

Question

I have a list of terms showing in slug order but I need to order them by most recent posts, so it will be changing depending on what term has the most recent entries... Is there a way to do so? I tried the solution proposed here: Retrieve taxonomy terms in order of their post's date?

But it is showing the terms in date of term creation, not the date of the most recent posts.

My code:

function wpse147412_order_terms_by_post_date( $pieces, $taxonomies, $args ) {
  global $wpdb;

  if ( 'post_date' !== $args['orderby'] ) {
    return $pieces;
  }

  $args = wp_parse_args( $args, array( 'post_types' => 'episode' ) );

  $pieces['fields']   = 'DISTINCT ' . $pieces['fields'];
  $pieces['join']    .= " JOIN $wpdb->term_relationships AS tr ON tr.term_taxonomy_id =   tt.term_taxonomy_id";
  $pieces['join']    .= " JOIN $wpdb->posts AS p ON p.ID = tr.object_id";
  $pieces['where']   .= " AND p.post_type IN ('" . implode( "', '", (array)   $args['post_types'] ) . "')";
  $pieces['orderby']  = 'ORDER BY p.post_date';

  return $pieces;
}
add_filter( 'terms_clauses', 'wpse147412_order_terms_by_post_date', 10, 3 );

Placed above in functions, then in the page I need the terms:

<?php 
$tax_terms = get_terms( 'series',
  array( 'post_types' => 'episode',
    'orderby' => 'post_date',
    'order' => 'DESC' ) 
  );

if( is_array( $tax_terms ) && count( $tax_terms ) > 0 ):
  foreach ($tax_terms as $tax_term):
  ?>

    <div class="series-button text-center">
      <a href="<?php echo esc_attr(get_term_link($tax_term, $taxonomy));?>">
        <figure class="align-items-center d-flex" style="background-image: url('<?php echo z_taxonomy_image_url($tax_term->term_id, 'series-img'); ?>');">  
          <h3 class="text-center uppercase"><?php echo $tax_term->name; ?></h3>
        </figure> 
      </a>
    </div>

  <?php 
  endforeach;
endif;
?>

Answer 1

This will not be the fastest way to achieve it, - but it will do the trick. If you have a bazillion posts (I'd say more than 1000) - or if you're server is slow, then this might be too heavy. But I wouldn't hesitate to use it on any of my sites.

If you want something fast, then you should probably go with some SQL of some sort.

<?php
$episode_query = new WP_Query([
  'post_type' => 'post',
  'post_status' => 'publish',
  'posts_per_page' => 999,
  'order' => 'DESC',
  'orderby' => 'date'
]);

$terms = [];
if( $episode_query->have_posts() ):
  while( $episode_query->have_posts() ):
    $episode_query->the_post();
    $post = get_post();

    // Tags
    $tags = get_the_terms( $post, 'post_tag' );
    if( !empty( $tags ) ):
      foreach( $tags as $tag ):
        if( does_array_contain_term( $tag, $terms ) ){
          $terms[] = $tag;
        }
      endforeach; // foreach( $tags as $item ):
    endif; // if ( !empty( $tags ) )

    // Category
    $categories = get_the_terms( $post, 'category' );
    if( !empty( $categories ) ):
      foreach( $categories as $cat ):
        if( does_array_contain_term( $cat, $terms ) ){
          $terms[] = $cat;
        }
      endforeach; // foreach( $categories as $item ):
    endif; // if ( !empty( $categories ) )


  endwhile; // while( $episode_query->have_posts() ):
  wp_reset_query();
endif; // if( $episode_query->have_posts() ):

function does_array_contain_term( $term_to_check, $terms ){
  $add_term = true;
  foreach( $terms as $term ){
    if( $term->slug == $term_to_check->slug ){
      $add_term = false;
    }
  }

  return $add_term;
}

// TO SEE THE RESULT
echo '<pre>';
print_r($terms);
echo '</pre>';
?>