Implement the GROUP BY feature

The GROUP BY is missing
Datajar syntax proposed :

$query = Post::query()
                            ->where(array('key' => $this->user->getLogin(), 'parentid' => ''))
                            ->orderby('updated', true)
                            ->groupby('jid') // or ->groupby(array('jid', 'key'))
                            ->join('Attachment', array('key_src' => 'key_dest');
$messages = Post::run_query($query);