public function isAckAction()
    {
        $json = array();
        $postId = $this->params()->fromQuery('postid', null);
        $myReactions = $this->mattermost->getMyReactions($postId);
        $ok = array_filter($myReactions, function($v){
            return strcmp($v['emoji_name'], 'ok') == 0;
        });
        $json['ack'] = count($ok) == 1;
        return new JsonModel($json);
    }