public function getLastPost($pk)
{
$mapper = new RelationDataMapper('post', Table::POSTS);
$mapper->addTable('topic', Table::TOPICS, 'topic.id = post.topic_id')->addTable('category', LunaTable::CATEGORIES, 'category.id = topic.category_id')->addTable('user', WarderTable::USERS, 'user.id = post.user_id');
$post = $mapper->findOne(array('category.id' => $pk), 'post.created DESC');
return $post;
}