public function getFieldName($k, $alias= null) {
        if ($this->fieldNames === null) {
            $this->_initFields();
        }
        $name= null;
        $k = $this->getField($k, true);
        if (is_string($k) && isset ($this->fieldNames[$k])) {
            $name= $this->fieldNames[$k];
        }
        if ($name !== null && $alias !== null) {
            $name= str_replace("{$this->_table}.", "{$alias}.", $name);
        }
        return $name;
    }