code: private function getStockUnitMapper(EntityManagerInterface $em)
      {
          if (null === $this->stockUnitMapper) {
              $this->stockUnitMapper = new DiscriminatorMapper($em, AbstractStockUnit::class);
          }
  
          return $this->stockUnitMapper;
      }
docs: Returns the stock unit mapper.
  
  @param EntityManagerInterface $em
  
  @return DiscriminatorMapper