protected function _createFilter()
    {
        $config = array();
        $config['separator'] = $this->_separator;

        if (!isset($this->_length)) {
            $config['length'] = $this->getTable()->getColumn('slug')->length;
        } else {
            $config['length'] = $this->_length;
        }

        return $this->getObject('filter.factory')->createChain($this->_filter, $config);
    }