Django模型方法无法访问信号赋值的__original_instance属性

定义了Django模型SrdRevisionWording，其中__original_instance是自定义非数据库属性，用于存储历史实例。在pre_save信号中为该属性赋值后，信号内打印显示已成功获取实例对象；post_save信号中也能正常打印该属性，但调用模型的get_revision_history_message方法时，出现AttributeError，提示__original_instance为None，无法访问其word属性。

模型代码 #

class SrdRevisionWording(ModelDiff):
    srd_revision = models.ForeignKey(SrdRevision, on_delete=models.CASCADE)
    word = models.CharField(max_length=50, null=False)
    designation = models.CharField(max_length=200, null=False)
    is_list = models.BooleanField(null=False, default=False)
    parent_element = models.IntegerField(null=True) # 上一SRD版本中Wording元素的ID
    __original_instance = None
    with_signal = True

    def get_revision_history_message(self, created=False, deleted=False, updated_field=None, check_history=False):
        if created:
            return 'Word "{}" added'.format(self.word)

        if deleted:
            return 'Word "{}" deleted'.format(self.word)

        match updated_field:
            case 'word':
                return 'Word {} changed to {}'.format(self.__original_instance.word, self.word)
            case 'designation' | 'is_list':
                return 'Word {} modified'.format(self.word)

pre_save信号代码 #

@receiver(pre_save, sender=SrdRevisionDocument)
@receiver(pre_save, sender=SrdRevisionWording)
def model_pre_save_handler(sender, instance, **kwargs):
    print('pre_save')
    if instance.srd_revision.revision != 'A':
        try:
            previous_srd_revision_state = sender.objects.get(pk=instance.parent_element)
        except sender.DoesNotExist:
            instance.__original_instance = None
        else:
            instance.__original_instance = previous_srd_revision_state
        print(instance.__original_instance)

post_save信号代码 #

@receiver(post_save, sender=SrdRevisionDocument)
@receiver(post_save, sender=SrdRevisionWording)
def handle_revision_history_element_changed(sender, instance, **kwargs):
    print('signal')
    print(instance.__original_instance)
    if instance.with_signal is not False:
       message = instance.get_revision_history_message(created=True, check_history=True)

报错信息 #

return 'Word {} changed to {}'.format(self.__original_instance.word, self.word)
                                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'word'

核心原因 #

1. Django实例的数据库刷新机制：post_save信号触发时，Django会将刚保存的实例从数据库重新加载，而__original_instance是内存临时属性，不会被持久化到数据库，刷新后的实例会丢失该属性，恢复为类定义的默认值None。
2. 信号打印的误导性：post_save中打印instance.__original_instance时，可能实例还未完成数据库刷新，但调用get_revision_history_message时，实例已完成刷新，属性已丢失。
3. 方法调用参数错误：post_save中调用get_revision_history_message时传入了created=True，但报错显示走到了word更新分支，说明实际调用时created可能为False或updated_field被设为word，此时__original_instance已变为None。

解决方案 #

1. 存储必要字段而非完整实例：在pre_save中只存储需要的历史字段值，避免依赖完整实例：

   @receiver(pre_save, sender=SrdRevisionDocument)
   @receiver(pre_save, sender=SrdRevisionWording)
   def model_pre_save_handler(sender, instance, **kwargs):
       print('pre_save')
       if instance.srd_revision.revision != 'A':
           try:
               previous_srd_revision_state = sender.objects.get(pk=instance.parent_element)
               instance._original_word = previous_srd_revision_state.word
           except sender.DoesNotExist:
               instance._original_word = None
           print(instance._original_word)
   

   对应修改get_revision_history_message：

   case 'word':
       return 'Word {} changed to {}'.format(self._original_word, self.word)
   

2. 在方法内直接查询历史数据：不再依赖临时属性，在get_revision_history_message内部查询历史实例：

   def get_revision_history_message(self, created=False, deleted=False, updated_field=None, check_history=False):
       if created:
           return 'Word "{}" added'.format(self.word)
   
       if deleted:
           return 'Word "{}" deleted'.format(self.word)
   
       original_word = self.word
       if updated_field == 'word' and self.parent_element is not None:
           try:
               original_instance = self.__class__.objects.get(pk=self.parent_element)
               original_word = original_instance.word
           except self.__class__.DoesNotExist:
               pass
   
       match updated_field:
           case 'word':
               return 'Word {} changed to {}'.format(original_word, self.word)
           case 'designation' | 'is_list':
               return 'Word {} modified'.format(self.word)
   

3. 修正方法调用参数：根据post_save的kwargs传递正确参数：

   def handle_revision_history_element_changed(sender, instance, **kwargs):
       print('signal')
       if instance.with_signal is not False:
           message = instance.get_revision_history_message(
               created=kwargs.get('created', False),
               updated_field='word'  # 根据实际业务逻辑传递正确字段
           )
   

内容的提问来源于stack exchange，提问作者Atro_TLT