code: public function getKeys(): array {
  		if (1 == count($this->dbtextCollections)) {
  			return reset($this->dbtextCollections)->getKeys();
  		}
  		
  		$keys = array();
  		
  		foreach ($this->dbtextCollections as $dbtextCollection) {
  			array_push($keys, ...$dbtextCollection->getKeys());
  		}
  		
  		return array_unique($keys);;
  	}
docs: {@inheritDoc}
  @see \dbtext\model\DbtextCollection::getKeys()