code: private function getSubjectIdentityMapper(EntityManagerInterface $em)
      {
          if (null === $this->subjectIdentityMapper) {
              $this->subjectIdentityMapper = new EmbeddableMapper($em, SubjectIdentity::class);
          }
  
          return $this->subjectIdentityMapper;
      }
docs: Returns the subjectIdentityMapper.
  
  @param EntityManagerInterface $em
  
  @return EmbeddableMapper