docs: def change_parent(sender, instance, **kwargs):
      """
      When the given flashcard has changed. Look at term and context and change
      the corresponding item relation.
      """
      if instance.id is None:
          return
      if len({'term', 'term_id'} & set(instance.changed_fields)) != 0:
          diff = instance.diff
          parent = diff['term'][0] if 'term' in diff else diff['term_id'][0]
          child_id = instance.item_id
          if parent is not None:
              parent_id = parent.item_id if isinstance(parent, Term) else Term.objects.get(pk=parent).item_id
              ItemRelation.objects.filter(parent_id=parent_id, child_id=child_id).delete()
          ItemRelation.objects.get_or_create(parent_id=instance.term.item_id, child_id=child_id, visible=True)
      if len({'term_secondary', 'term_secondary_id'} & set(instance.changed_fields)) != 0:
          diff = instance.diff
          child_id = instance.item_id
          parent = diff['term_secondary'][0] if 'term_secondary' in diff else diff['term_secondary_id'][0]
          if parent is not None:
              parent_id = parent.item_id if isinstance(parent, Term) else Term.objects.get(pk=parent).item_id
              ItemRelation.objects.filter(parent_id=parent_id, child_id=child_id).delete()
          if instance.term_secondary is not None or instance.term_secondary_id is not None:
              ItemRelation.objects.get_or_create(parent_id=instance.term_secondary.item_id, child_id=child_id, visible=True)
      if len({'context', 'context_id'} & set(instance.changed_fields)) != 0:
          diff = instance.diff
          parent = diff['context'][0] if 'context' in diff else diff['context_id'][0]
          child_id = instance.item_id
          if parent is not None:
              parent_id = parent.item_id if isinstance(parent, Context) else Context.objects.get(pk=parent).item_id
              ItemRelation.objects.filter(parent_id=parent_id, child_id=child_id).delete()
          ItemRelation.objects.get_or_create(parent_id=instance.context.item_id, child_id=child_id, visible=True)