code: def get_all_children(self, include_self=False):
          
          ownership = Ownership.objects.filter(parent=self)
          subsidiaries = Company.objects.filter(child__in=ownership)
          for sub in subsidiaries:
              subsidiaries = subsidiaries | sub.get_all_children()
  
          if include_self is True:
              self_company = Company.objects.filter(id=self.id)
              subsidiaries = subsidiaries | self_company
  
          return subsidiaries
docs: Return all subsidiaries of this company.