public function getQueryFields($usePrefix = false, $prefix = '')
    {
        $fields = $this->fields;

        if ($usePrefix) {
            $prefix = (empty($prefix) ? $this->getTable() : $prefix);
            $fields = array();
            foreach ($this->fields as $field) {
                $fields[] = $prefix . '.' . $field;
            }
        }

        return implode(', ', $fields);
    }