code: def save(self, must_create=False):
          
          if self.session_key is None:
              return self.create()
          data = self._get_session(no_load=must_create)
          obj = self.create_model_instance(data)
          obj.save()
docs: Saves the current session data to the database. If 'must_create' is
          True, a database error will be raised if the saving operation doesn't
          create a *new* entry (as opposed to possibly updating an existing
          entry).
  
          :param must_create: