public function getDefaultGroup()
    {
        $aGroups = $this->getAll([
            'where' => [
                ['is_default', true],
            ],
        ]);

        if (empty($aGroups)) {
            throw new NailsException('A default user group must be defined.');
        }

        $this->oDefaultGroup = reset($aGroups);

        return $this->oDefaultGroup;
    }