如何测试Django post_save信号中事务提交后的执行逻辑

你无法触发else分支不是Django测试事务机制导致的，是你判断「新建/更新对象」的业务逻辑本身存在错误：

• Django post_save 信号的触发时机是模型save()方法完成数据库INSERT/UPDATE操作之后
• 同事务内的查询可以读到当前事务未提交的写入数据，因此只要进入post_save逻辑，Campaign.objects.filter(pk=instance.pk).exists()永远返回True，哪怕事务还没提交
• 你写的else分支在生产环境也永远不会执行，和测试环境无关

Django的post_save信号自带created关键字参数，值为True时代表当前是新建对象的场景，值为False代表是更新已有对象的场景，不需要手动查库判断，修正后的信号代码如下：

@receiver(post_save, sender=Campaign, dispatch_uid="apps.writing.signals.create_handwriting")
def create_handwriting(sender, instance, created, **kwargs):
    """新建或更新活动时，触发手写云函数（重新）生成手写图片"""
    if not created:
        transaction.on_commit(
            lambda: log_campaign_progress(pk=instance.pk, status="t2h-edited", stage="campaign")
        )
        transaction.on_commit(lambda: delete_campaign_pages(campaign_pk=instance.pk))
    else:
        transaction.on_commit(
            lambda: log_campaign_progress(pk=instance.pk, status="t2h-created", stage="campaign")
        )

    transaction.on_commit(lambda: enqueue_handwriting_generation(campaign_pk=instance.pk))

你已经用到的captureOnCommitCallbacks(execute=True)是Django TestCase专门提供的、用于测试transaction.on_commit逻辑的工具，不需要切换为速度更慢的TransactionTestCase，只需要把新建、更新两个场景拆分为独立测试用例即可：

class TestSignals(TestCase):
    def setUp(self):
        self.factory = RequestFactory()
        # 测试前连接信号
        signals.post_save.connect(
            sender=Campaign,
            dispatch_uid="apps.writing.signals.create_handwriting",
            receiver=create_handwriting,
        )
        # 测试结束后自动断开信号，避免影响其他用例
        self.addCleanup(
            signals.post_save.disconnect,
            sender=Campaign,
            dispatch_uid="apps.writing.signals.create_handwriting",
        )

    @mock.patch("lettergun.apps.writing.signals.log_campaign_progress")
    @mock.patch("lettergun.apps.writing.signals.enqueue_handwriting_generation")
    @mock.patch("lettergun.apps.writing.signals.delete_campaign_pages")
    def test_new_campaign_triggers_create_logic(
        self, delete_campaign_pages, enqueue_handwriting_generation, log_campaign_progress
    ):
        enqueue_handwriting_generation.return_value = True
        log_campaign_progress.return_value = True

        with self.captureOnCommitCallbacks(execute=True):
            user = G(User)
            campaign = G(Campaign, user=user)

        # 断言新建场景逻辑调用正确
        enqueue_handwriting_generation.assert_called_once_with(campaign_pk=campaign.pk)
        log_campaign_progress.assert_called_once_with(pk=campaign.pk, stage="campaign", status="t2h-created")
        # 新建场景不会触发删除已有页面的逻辑
        delete_campaign_pages.assert_not_called()

    @mock.patch("lettergun.apps.writing.signals.log_campaign_progress")
    @mock.patch("lettergun.apps.writing.signals.enqueue_handwriting_generation")
    @mock.patch("lettergun.apps.writing.signals.delete_campaign_pages")
    def test_edited_campaign_triggers_update_logic(
        self, delete_campaign_pages, enqueue_handwriting_generation, log_campaign_progress
    ):
        enqueue_handwriting_generation.return_value = True
        log_campaign_progress.return_value = True
        delete_campaign_pages.return_value = True
        # 提前创建测试活动
        user = G(User)
        campaign = G(Campaign, user=user)
        # 重置mock，清除创建活动时产生的调用记录
        log_campaign_progress.reset_mock()
        enqueue_handwriting_generation.reset_mock()
        delete_campaign_pages.reset_mock()

        with self.captureOnCommitCallbacks(execute=True):
            # 修改并保存已有活动，触发更新场景
            campaign.title = "修改后的活动标题"
            campaign.save()

        # 断言更新场景逻辑调用正确
        enqueue_handwriting_generation.assert_called_once_with(campaign_pk=campaign.pk)
        log_campaign_progress.assert_called_once_with(pk=campaign.pk, stage="campaign", status="t2h-edited")
        delete_campaign_pages.assert_called_once_with(campaign_pk=campaign.pk)

• 测试更新场景时，必须在创建完测试对象后重置mock，否则创建对象时触发的信号调用会导致断言失败
• 如果使用factory_boy生成测试数据，确认工厂类没有默认断开post_save信号，否则信号逻辑不会触发
• 不要通过事务内查库的方式判断对象是否为新建，同事务内可以读到未提交的写入，这类判断逻辑永远不会生效

内容的提问来源于stack exchange，提问作者John