I created a custom where clause. I tested it using $wp_query->request right before my main loop, I don't really know SQL that well, but this seemed to get things working.
add_action('pre_get_posts', 'add_trending_sort', 11, 1);
function add_trending_sort($query){
if(!$query->is_main_query())
return;
//Overwrite query arguments
$query->set('meta_query', array(
array(
'key' => 'TRENDING',
//'value' => 'asdfasdf',//may need a value for older versions of WordPress
'compare' => 'NOT EXISTS',
)
));
$query->set('orderby', 'meta_value_num date');
$query->set('order', 'DESC');
}
add_filter('posts_where', 'add_trending_where');
function add_trending_where($where = ''){
global $wpdb, $wp_query;
if(!$wp_query->is_main_query())//Not sure if this really works. Should be OK
return $where;
$where .= " OR ( $wpdb->postmeta.meta_key = 'TRENDING' )";
// Don't run this twice
remove_filter('posts_where', 'add_trending_where');
return $where;
}
Alternatively, you could set compare to 'EXISTS' and change the line in add_trending_where to $where .= " OR ($wpdb->postmeta.post_id IS NULL)";. Then you'd only have to change the value of the key in one place. Again, echo $wp_query->request and play around if you want understand this better or tweak it.
EDIT: I just noticed this does not work if meta_key is set on the query. You could use $query->set('meta_key', NULL); if you have to.
EDIT 2: I got this working with the method above. For some reason it wasn't at first (maybe meta_key was set... I don't know).
add_action('pre_get_posts', 'add_trending_sort', 11, 1);
function add_trending_sort($query){
// Bail if not the main "hidden" query, as opposed to a 'new WP_Query()' call
if(!$query->is_main_query())
return;
// Set meta_query to get shares for orderby, and also get non-shared content.
$query->set('meta_query', array(
'relation' => 'OR',
array(
'key' => 'TRENDING',
'compare' => 'NOT EXISTS',
),
array(
'key' => 'TRENDING',
'compare' => 'EXISTS',
)
));
//$query->set('meta_key', NULL);
$query->set('orderby', array('meta_value_num' => 'DESC', 'date' => 'DESC'));
}