Django ORM - 获取组的最新记录

问题描述

假设我们有一个 Django ORM 模型 Meetup，其定义如下:

class Meetup(models.Model):语言 = 模型.CharField()日期 = 模型.DateField(auto_now=True)

我想获取每种语言的最新聚会.

看来您可以使用 Django Aggregates 来实现这一点查找简单:

Meetup.objects.annotate(latest_date=Max("date")).values("language", "latest_date")

在我看来，这应该获取每种语言的最新"聚会.但事实并非如此:

>>>Meetup.objects.create(language='python')<聚会:聚会对象>>>>Meetup.objects.create(language='python')<聚会:聚会对象>>>>Meetup.objects.create(language='node')<聚会:聚会对象>>>>Meetup.objects.create(language='node')<聚会:聚会对象>>>>Meetup.objects.annotate(latest_date=Max("date")).values("language", "latest_date").count()4

我希望能得到两个最新的 Python 和 Node 聚会！

如何构建一个查询，只获取每种语言的最新聚会?

附注.我使用 MySQL 作为我的后端.

解决方案

把你的 values 子句放在 annotate 之前.

来自 聚合文档:

如果 values() 子句在 annotate() 之前，将使用 values() 子句描述的分组来计算注释.

但是，如果 annotate() 子句在 values() 子句之前，注释将在整个查询集上生成.在这种情况下，values() 子句仅约束在输出时生成的字段.

所以应该这样做:

Meetup.objects.values('language').annotate(latest_date=Max('date'))

Imagine we have the Django ORM model Meetup with the following definition:

class Meetup(models.Model):
    language = models.CharField()
    date = models.DateField(auto_now=True)

I'd like to fetch the latest meetup for each language.

It would seem you could use Django Aggregates to make this lookup easy:

Meetup.objects.annotate(latest_date=Max("date")).values("language", "latest_date")

In my mind this should fetch the "latest" meetup for each language. But that's not the case:

>>> Meetup.objects.create(language='python')
<Meetup: Meetup object>
>>> Meetup.objects.create(language='python')
<Meetup: Meetup object>
>>> Meetup.objects.create(language='node')
<Meetup: Meetup object>
>>> Meetup.objects.create(language='node')
<Meetup: Meetup object>
>>> Meetup.objects.annotate(latest_date=Max("date")).values("language", "latest_date").count()
4

I expected to get just the two latest Python and Node meetups!

How can I construct a query that will fetch only the latest meetups for each language?

PS. I'm using MySQL as my backend.

解决方案

Put your values clause before the annotate.

From the aggregation docs:

So this should do it:

Meetup.objects.values('language').annotate(latest_date=Max('date'))

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