如何在Django中预取聚合的@property？ [英] How to prefetch aggregated @property in Django?

问题描述

我们有两个模型（简化版本）：

We have two models (simplified versions):

class Contestant(models.Model):
    email = models.EmailField(max_length=255, unique=True)
    # plus some other fields

    @property
    def total_points(self):
        return self.points.aggregate(total=Sum('value'))['total'] or 0

class Points(models.Model):
    contestant = models.ForeignKey(Contestant, related_name='points')
    value = models.PositiveIntegerField()
    # plus some other fields which determine based on what we
    # awarded ``Points.value``

当我们显示参赛者名单以及 total_points 值时，它
导致对每个结果的额外查询 - 即执行以下查询：

When we display a list of contestants along with their total_points value, it results in an extra query for each result - i.e. the following queries are performed:

1. 获取参赛者列表


2. 获取 total_points 第一参赛者的价值


3. 获取 total_points v第二名参赛者的评价


4. 等

1. fetch list of contestants
2. fetch total_points value of 1st contestant
3. fetch total_points value of 2nd contestant
4. etc

我尝试改变查询集，以便如下预取数据：

I tried altering the queryset to prefetch the data as follows:

Contestant.objects.filter(...).prefetch_related('points')

...，但是即使它有效，当
列出参赛者时，预取数据也不会被使用（所以每个结果仍然会尝试在单独的查询中获取 total_points
。

..., however even though it works, the prefetched data is not utilized when listing contestants (so each result still tries to fetch total_points in a separate query).

是否可以：

• 以
  为单个模型对象填充数据，以某种方式告诉ORM为@property字段使用预取值（例如访问 Contestant.total_points @property方法中的预取值）


• 或以不同的方式预取它们（而不是或者使用完全不同的方法实现相同的结果？

• somehow tell the ORM to use prefetched values for the @property field when populating data for individual model objects (e.g. access the prefetched value inside the Contestant.total_points @property method)?
• or to prefetch them in a different way (as opposed to the example above)?
• or to use a completely different approach achieving the same result?

（我在 tastypie 中列出结果，如果重要）。

(I'm listing results in tastypie, if it matters.)

谢谢。

推荐答案

当您的目的是为每个项目添加聚合值时，您应该使用 annotate / a>，而不是聚合。

When your aim is to add aggregated values to each item, you should use annotate, instead of aggregate.

例如（一个简单的查询，不需要额外的方法）：

For example (a simple query, no additional methods required):

Contestant.objects.filter(...).annotate(total_points=Sum('points__value'))

---

如果你真的想把这个代码放出您的查询：您可以，但是一种模型方法不是正确的方法。模型上的方法用于单个实例的操作。如果您想在整个QuerySet上执行某些操作，请使用 ORM管理器

有一个经理这样看起来像这样：

With a Manager this would look like this:

class TotalPointsManager(models.Manager):
    def get_queryset(self):
        return super(TotalPointsManager, self).get_queryset().annotate(total_points=Sum('points__value'))

class Contestant(models.Model):
    email = models.EmailField(max_length=255, unique=True)

    objects = TotalPointsManager() # You are overriding the default manager!

然后你将像往常一样构造你的查询（你可以删除 prefetch_related ）：

and then you would construct your query as usual (you can drop prefetch_related):

Contestant.objects.filter(...)

...和 total_points 字段将成为神奇可用于每个对象。

...and total_points field would become "magically" available for every object.

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