相关疑难解决方法(0)

我有以下型号:

class City(models.Model):
    ...

class Census(models.Model):
    city = models.ForeignKey(City)
    date = models.DateTimeField()
    value = models.BigIntegerField()

现在我想用最新的人口普查值来注释City-queryset.我如何实现这一目标？

我试过了:

City.objects.annotate(population=Max('census__date'))
# --> annotates date and not value

City.objects.annotate(population=Max('census__value'))
# --> annotates highest value, not latest

City.objects.annotate(population=
    Case(
        When(
            census__date=Max('census__date'),
            then='census__value')
        )
    )

# --> annotates 'None'

City.objects.annotate(population=
    Case(
        When(
            census__date=Max('census__date'),
            then='census__value')
        ), output_field=BigIntegerField()
    )

# --> takes forever (not sure what happens at the end, after some minutes I stopped waiting)

任何帮助非常感谢!

如何在单个表达式中从多对一关系的过滤结果中获取最新值以注释 Django QuerySet？

from django.db import models

class Lorem(models.Model):
    """ Lorem ipsum, dolor sit amet. """

class LoremStatusEvent(models.Model):
    """ A status event change on a given `ipsum` for a `lorem`. """

    created = models.DateTimeField(auto_now_add=True)
    lorem = models.ForeignKey(Lorem)
    ipsum = models.CharField(max_length=200)
    status = models.CharField(max_length=10)