protected function onBeforeDelete()
{
parent::onBeforeDelete();
foreach ($this->Votes() as $e) {
$e->delete();
}
foreach ($this->Comments() as $e) {
$e->delete();
}
foreach ($this->ChangeRequests() as $e) {
$e->delete();
}
foreach ($this->Materials() as $e) {
$e->delete();
}
$this->Topics()->removeAll();
$this->Speakers()->removeAll();
}
