code: def apply_chord(self, header, partial_args, group_id, body, **options):
          u
          callback_entry = TaskMeta.objects.create(task_id=body.id)
          chord_data = ChordData.objects.create(callback_result=callback_entry)
          for subtask in header:
              subtask_entry = TaskMeta.objects.create(task_id=subtask.id)
              chord_data.completed_results.add(subtask_entry)
          if body.options.get(u'use_iterator', None) is None:
              body.options[u'use_iterator'] = True
          chord_data.serialized_callback = json.dumps(body)
          chord_data.save()
  
          return header(*partial_args, task_id=group_id)
docs: u"""
          Instantiate a linking ChordData object before executing subtasks.
  
          Parameters
          ----------
              header: a list of incomplete subtask signatures, with partial
                  different-per-instance arguments already set.
              partial_args: list of same-per-instance subtask arguments.
              group_id: a uuid that proved unnecessary in our approach. We use
                  the callback's frozen TaskMeta id as a linking piece of data.
              body: the callback task signature, with all non-subtask-dependent
                  arguments already set.
  
          Return value is the (unfinished) AsyncResult for body.