protected function _where($condition, $value = null, $type = null, $bool = true)
    {
        if (count($this->_parts[self::UNION])) {
            // require_once 'Zend/Db/Select/Exception.php';
            throw new Zend_Db_Select_Exception("Invalid use of where clause with " . self::SQL_UNION);
        }

        if ($value !== null) {
            $condition = $this->_adapter->quoteInto($condition, $value, $type);
        }

        $cond = "";
        if ($this->_parts[self::WHERE]) {
            if ($bool === true) {
                $cond = self::SQL_AND . ' ';
            } else {
                $cond = self::SQL_OR . ' ';
            }
        }

        return $cond . "($condition)";
    }