如何在datetime字段上过滤对象？

class Announcement(models.Model):
    name = models.CharField(max_length=500)
    content = models.CharField(max_length=1000)
    is_active = models.BooleanField(default=False)
    date_start = models.DateTimeField()
    date_end = models.DateTimeField()

    def __str__(self):
        return self.name

我可以过滤is_active字段上的对象,如：

def announcements(request):
    announcements = Announcement.objects.all().filter(
        is_active = True
    )
    return HttpResponse(
        serializers.serialize("json",announcements))

这很好用.但是当我想基于date_start字段过滤它们时,如下所示：

def announcements(request):
    announcements = Announcement.objects.all().filter(
        date_start >= datetime.now()
    )
    return HttpResponse(
        serializers.serialize("json",announcements))

我得到全局名称’date_start’没有定义错误.