使用条件注释在Django 1.8中使用最新相关的值进行注释 [英] Annotate with value of latest related in Django 1.8 using conditional annotation

问题描述

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标注城市查询。我如何实现？

Now I'd like to annotate a City-queryset with the value of the latest Census. How do I achieve that?

我尝试过：

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)

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推荐答案

我也遇到一个需要相关集的最大值对象的问题，但我需要整个对象。我无法找出使用注释和案例的解决方案。直到我这样做，我使用这个预取解决方案。如果每个城市没有大量的人口普查对象，或者如果您的申请没有履行约束力，这可能适用于您。

I've also been having an issue where I need a max value object of a related set, but I require the entire object. I have not been able to figure out a solution using annotation and Case. Until I do, I use this prefetching solution. If each city does not have a large amount of census objects, or if your application is not performance bound, this may work for you.

inner_qs = Census.objects.order_by('-date')
cities = City.objects.prefetch_related(Prefetch("census_set", queryset=inner_qs, to_attr="census_list"))

class City(models.Model):
    @property
    def latest_census(self):
        if hasattr(self, 'census_list') and len(self.census_list) > 0:
            return self.census_list[0]
        return None

如果不适合你，请考虑以下一些建议：
http://blog.roseman.org.uk/2010/08/14/getting-related-item-aggregate/

If this does not work for you, consider some of the suggestions found here: http://blog.roseman.org.uk/2010/08/14/getting-related-item-aggregate/

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