在 Django 中使用 F() 和 timedelta 进行查询 [英] Making queries using F() and timedelta at django

问题描述

我有以下模型:

class Process(models.Model):
  title = models.Charfield(max_length=255)
  date_up = models.DateTimeField(auto_now_add=True)
  days_activation = models.PositiveSmallIntegerField(default=0)

现在我需要根据 days_activation 的值查询所有已过期的 Process 对象.

Now I need to query for all Process objects that have expired, according to their value of days_activation.

我试过了

from datetime import datetime, timedelta

Process.objects.filter(date_up__lte=datetime.now()-timedelta(days=F('days_activation')))

并收到以下错误消息:

TypeError: timedelta days 组件不支持的类型:F

我当然可以用 Python 来做:

I can of course do it in Python:

filter (lambda x: x.date_up<=datetime.now() - timedelta(days=x.days_activation), 
        Process.objects.all ()), 

但我真的需要生成一个 django.db.models.query.QuerySet.

but I really need to produce a django.db.models.query.QuerySet.

推荐答案

7 days == 1 day * 7

F 是深黑色的 Django 魔法和遇到它的对象必须属于相应的魔法阵才能处理.

7 days == 1 day * 7

F is deep-black Django magic and the objects that encounter it must belong to the appropriate magical circles to handle it.

在您的情况下，django.db.models.query.filter 知道 F，但 datetime.timedelta 不知道.因此，您需要将 F 保留在 timedelta 参数列表之外.幸运的是，F 支持timedelta * int 的乘法，所以以下可以工作:

In your case, django.db.models.query.filter knows about F, but datetime.timedelta does not. Therefore, you need to keep the F out of the timedelta argument list. Fortunately, multiplication of timedelta * int is supported by F, so the following can work:

Process.objects.filter(date_up__lte=datetime.now()-timedelta(days=1)*F('days_activation'))

事实证明，这个将适用于 PostgreSQL，但不适用于 SQlite(对于 timedelta，Django 1.11 仅支持 + 和 -，可能是因为相应的 SQLite 限制).

As it turns out, this will work with PostgreSQL, but will not work with SQlite (for which Django 1.11 only supports + and - for timedelta, perhaps because of a corresponding SQlite limitation).

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