Meta Query for posts with a meta key not set

Hi, how can I do fetch posts for which a custom meta key is NOT set ? (like in ticket : https://core.trac.www.ads-software.com/ticket/18158) ?

I did try with the parameter

'meta_query' => array(
				array(
					'key' => $this->meta_key,
					'meta_compare'=>'!=',
					'meta_value'=>'true'
				)
			)

But it’s not workind, and also with the ‘posts_where’ filter

$where .= " AND {$wpdb->postmeta}.meta_key!='{$this->meta_key}'";

not working either.. Thanks !