code: def get_role_keys(cls, unit_key):
          
          stack = Role.objects.filter(unit_id=unit_key).values_list('key', flatten=True)
          for unit_key in cls.objects.filter(parent_id=unit_key).values_list('key', flatten=True):
              stack.extend(cls.get_role_keys(unit_key))
          return stack
docs: :param unit_key: Parent unit key
          :return: role keys of subunits