public function from($class, $alias= '') {
        if ($class= $this->xpdo->loadClass($class)) {
            $alias= $alias ? $alias : $class;
            $this->query['from']['tables'][]= array (
                'table' => $this->xpdo->getTableName($class),
                'alias' => $alias
            );
        }
        return $this;
    }