复杂的注释在django查询集 [英] complex annotate on django query set

问题描述

我有一个问题，我无法从我的django视图中的一个复杂的 .annotate（）调用中获取所需的所有信息。这是我的模型：

I have an issue where I can't quite get all the information I need from a complex .annotate() call in my django view. Here's my model:

RECORD_STATUS = (
    (0, "Hidden"),
    (1, "Unhidden"),
    (2, "Deleted"),
)
class Activity(models.Model):
    event_date = models.DateTimeField()
    duration = models.IntegerField(default=0)
    username = models.CharField(max_length=200)
    record_id = models.CharField(max_length=100)
    record_status = models.IntegerField(choices=RECORD_STATUS)
    record_quality = models.FloatField(default=0)

我想通过基于我的视图模板的不同record_id的以下值：

I'd like to pass the following values based on a distinct record_id to my view template:

• 平均质量


• count


• 最新的event_date


• 总持续时间

• average quality
• count
• latest event_date
• total duration

..

...最新状态（特定record_id的最新event_date的活动状态）

...latest status (status of Activity with the latest event_date for a particular record_id)

我已经能够按照预期的方式获得大部分内容，并提供以下查询et codechunk但是我无法获取特定 record_id 的最新状态。我倾向于相信一个 .extra（）调用可能是要走的路，但我似乎无法正确构建它。

I've been able to get most of these as intended, with the following queryset codechunk but I am unable to grab the latest status for a particular record_id. I'm inclined to believe an .extra() call might be the way to go, but I can't seem to construct it properly.

record_id_details = Activity.objects.values(
    'record_id',
    ).annotate(
    ave_quality=Avg('record_quality'),
    count=Count('id'),
    latest_event_date=Max('event_date'),
    total_duration=Sum('duration'),
    # Here's where I'm going wrong
    # I need the record_statuses from the most recent Activities of each record_id
    latest_status=,# dont know... record_status WHERE Max(event_date) ???
).order_by(sort)

关于注释参数的任何建议，可以给我所需要的，或者也许一些帮助构建额外的命令将不胜感激。谢谢！

Any advice as to an annotate argument that would give me what I need, or perhaps some help constructing the extra command would be appreciated. Thanks!

推荐答案

没有内置的方式用Django查询表达这个查询。在两个查询中执行它可能是纯粹使用ORM最简单的方法。如果您不介意一些原始的SQL，并且您正在使用支持它的数据库（如PostgreSQL），那么您需要的工具称为Window函数，您可以在这里找到很好的细节： http://www.postgresguide.com/tips/window.html

There's no builtin way to express this query with a Django queryset. Performing it in two queries is probably the easiest way to do it purely with the ORM. If you don't mind some raw SQL, and you're using a database that supports it, like PostgreSQL, the tool you'll want is called a Window Function, you can find good detail here: http://www.postgresguide.com/tips/window.html

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