django 中的复杂过滤器方法

Question

我遇到了一个问题，我需要向所有培训在某个时间段（如 5/6 的比例）内到期的用户发送剩余电子邮件。我已经设置了一个调度系统，它将过滤所有每天到期的培训计划并将剩余的发送给他们。我面临的问题是我无法弄清楚查询。

这是我正在使用的模型：

class Training(models.Model):

    class Meta:
        ordering = ('-pk',)

    uuid = models.UUIDField(default=uuid.uuid4, editable=False)
    customer = models.ForeignKey('customer.Customer', on_delete=models.CASCADE, related_name='trainings')
    name = models.CharField(max_length=512)
    presentation = models.FileField(upload_to='trainings')
    data = models.JSONField()
    description = models.TextField(default='', blank=True)
    released = models.BooleanField(default=False)
    start_at = models.DateField()
    end_at = models.DateField()
    codes_file = models.FileField(null=True, max_length=1024)
    contact = models.EmailField(blank=True)
    is_archived = models.BooleanField(default=False)
    is_reminded = models.BooleanField(default=False)

这是我要运行的查询。

trainings = Training.objects.filter(is_reminded=False, ).annotate(
    deadline=(datetime.today() - F('start_at')).days / (F('start_at') - F('end_at')).days, output_field=DateField()
).filter(deadline__gt=0.833)

我从中得到的错误。我明白为什么会出现这个错误，但我不知道如何提取这些数据。

Internal Server Error: /testing/
Traceback (most recent call last):
  File "C:\Users\HP\.virtualenvs\apaa_backend-tCJa5vIy\lib\site-packages\django\core\handlers\exception.py", line 47, in inner
    response = get_response(request)
  File "C:\Users\HP\.virtualenvs\apaa_backend-tCJa5vIy\lib\site-packages\django\core\handlers\base.py", line 181, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "C:\Users\HP\.virtualenvs\apaa_backend-tCJa5vIy\lib\site-packages\django\views\decorators\csrf.py", line 54, in wrapped_view
    return view_func(*args, **kwargs)
  File "C:\Users\HP\.virtualenvs\apaa_backend-tCJa5vIy\lib\site-packages\rest_framework\viewsets.py", line 125, in view
    return self.dispatch(request, *args, **kwargs)
  File "C:\Users\HP\.virtualenvs\apaa_backend-tCJa5vIy\lib\site-packages\rest_framework\views.py", line 509, in dispatch
    response = self.handle_exception(exc)
  File "C:\Users\HP\.virtualenvs\apaa_backend-tCJa5vIy\lib\site-packages\rest_framework\views.py", line 469, in handle_exception
    self.raise_uncaught_exception(exc)
  File "C:\Users\HP\.virtualenvs\apaa_backend-tCJa5vIy\lib\site-packages\rest_framework\views.py", line 480, in raise_uncaught_exception
    raise exc
  File "C:\Users\HP\.virtualenvs\apaa_backend-tCJa5vIy\lib\site-packages\rest_framework\views.py", line 506, in dispatch
    response = handler(request, *args, **kwargs)
  File "C:\Users\HP\Desktop\apaa_backend\apaa\testing.py", line 22, in list_deadline_trainings
    deadline=(datetime.today() - F('start_at')).days / (F('start_at') - F('end_at')).days, output_field=DateField()
AttributeError: 'CombinedExpression' object has no attribute 'days'

我尝试了不同的解决方案来解决这个问题，比如使用 F() 注释一个新列，但无法弄清楚如何解决这个查询。如果有人可以帮助我或提出有用的建议，我将不胜感激。谢谢

Answer 1

F 表达式引用数据库中的数据并由数据库计算。它们不是 Python。

您也许可以使用Extract，但是一个真正复杂的查询集是否值得付出努力？

如果 Training 表的大小足够小或要提醒的分数足够大，请在查询集中执行您可以进行的过滤，其余的在 Python 中执行。

trainings = Training.objects.filter(is_reminded=False, )
    # maybe also filter end_at not in the past, start_at not in the future?
for training in trainings:
    deadline = (datetime.today() - training.start_at).days / (
        (training.start_at - training.end_at).days
    if deadline > 0.833:
        #do remind for this training
        training.is_reminded = True
        training.save()