private function swapPagePositions(Page $firstPage, Page $secondPage)
 {
     $firstPage->setPosition($firstPage->getPosition() + $secondPage->getPosition());
     $secondPage->setPosition($firstPage->getPosition() - $secondPage->getPosition());
     $firstPage->setPosition($firstPage->getPosition() - $secondPage->getPosition());
     $this->pageDao->update($firstPage);
     $this->pageDao->update($secondPage);
 }