Custom query which sorts posts based on last comment date

Question

I just want to sort posts from a standard query based on last comment date. I tried something like this (code below) but I can't get it right. PS: 'dzialy' => $cat this is custom taxonomy named dzialy and $cat is specified id of that category.

function forum_commentsjoin($join) {
 global $wp_query, $wpdb;
 if ($wp_query->query_vars['post_type']=='forum' && isset($wp_query->query_vars['dzialy'])) {
  $join .= "LEFT JOIN $wpdb->comments ON $wpdb->comments.comment_post_ID=$wpdb->posts.ID"; 
 } 
 return $join; 
}

...and later on...

$args = array( 'post_type' => 'forum', 'dzialy' => $cat, 'posts_per_page' => $ilosc, 'orderby'=>'comment_date', 'order'=>'DESC', );

print_r($loop->query); 

...after that…

Array ( [post_type] => forum [dzialy] => 1468 [posts_per_page] => 20 [orderby] => comment_date [order] => DESC )

Answer 1

You can use JOIN and wpdb class to run your raw sql queries

SELECT DISTINCT p.* 
FROM 
`wp_posts` p
LEFT JOIN  `wp_comments` c ON(p.`ID`=c.`comment_post_ID`)
WHERE p.`post_status`='publish'
AND p.`post_type`='forum'
ORDER BY c.`comment_date` DESC

Answer 2

try this code,

select wp_posts.*,
    coalesce(
        (
            select max(comment_date)
            from $wpdb->comments wpc
            where wpc.comment_post_id = wp_posts.id
        ),
        wp_posts.post_date
    ) as mcomment_date
    from $wpdb->posts wp_posts
    where post_type = 'post'
    and post_status = 'publish' 
    order by mcomment_date desc
    limit 10

Reference : Ordering Wordpress posts by most recent comment

Hope this helps you