Question

我有两个模型，如：

class Category(models.Model):
    title = models.CharField(max_length=100)
    expires_in_days = models.PositiveIntegerField(default=20)


class Item(models.Model):
    created = models.DateTimeField(auto_now_add=True)
    title = models.CharField(max_length=100)
    description = models.TextField()
    category = models.ForeignKey(Category)

我需要确定哪些项目应该设置过期：

items_to_cancel = Item.objects.annotate(expires_date=F('created') + F('category__expires_in_days'))\
                             .filter(expires_date__lte=timezone.now().date())

运行此查询会引发异常：

FieldError: Expression contains mixed types. You must set output_field.

更新

将查询更新为

后

items_to_cancel = Item.objects.annotate(expires_date=ExpressionWrapper(F('created') + F('category__expires_in_days'), output_field=DateField()))\
                             .filter(expires_date__lte=timezone.now().date())

我遇到了这个例外：

ProgrammingError: operator does not exist: timestamp with time zone + integer.

所以现在我无法弄清楚如何从时间戳中获取日期。

Answer 1

感谢Daniel Roseman对ExpressionWrapper的评论。但是我必须再做一次更新才能使它工作，我必须将TruncDate函数应用于时间戳：

from django.db.models.functions import TruncDate

items_to_cancel = Item.objects.annotate(
        expires_date=ExpressionWrapper(
            TruncDate('created') + F('category__expires_in_days'),
            output_field=DateField()
        )
    ).filter(expires_date__lte=timezone.now().date())

如何创建包含不同类型的queryset表达式

1 个答案: