django最有效的方法来计算查询中的相同字段值 [英] django most efficient way to count same field values in a query

问题描述

如果我有一个模型有很多字段，但我只关心一个字段。让我们说charfield可以是任何东西，所以我不知道可能的值，但是我知道这些值经常重叠。所以我可以有20个对象与abc和10个对象与xyz或我可以有50个对象与def和80与stu，我有40000没有重叠，我真的不在乎。

如何有效地计数对象？我想要返回的是：

{'abc'：20，'xyz'：10，'other'：10,000}

或类似的东西，w / o做一吨的SQL调用。

编辑：

我不知道有人会看到这个，因为我正在编辑它迟到了， ...

我有这个模型：

 
 class Action（models。模型）：
 author = models.CharField（max_length = 255）
 purl = models.CharField（max_length = 255，null = True）
 

，从答案中我已经这样做了：

 
 groups = Action.objects.filter（author = （'purl'）。注释（count = Count（'purl'））
 

p>

这是什么组：

 
 {purl：waka} ，{purl：waka}，{purl：waka}，{purl：waka}，{purl：mora}，{purl ，{purl：mora}，{purl：mora}，{purl：mora}，{purl：lora} 
 

（我只是用虚拟值填充purl）

我想要的是

 
 {'waka'：4，'mora'：5，'lora'：1} 
 

何仔细看有人会看到这个编辑...

编辑2：

显然我的数据库（BigTable）支持Django的聚合功能，这就是为什么我一直遇到所有问题。

解决方案

你想要一些类似于count ... 通过...分组。您可以使用django的ORM的聚合功能：

 从django.db.models导入计数
 
 fieldname ='myCharField'
 MyModel.objects.values（fieldname）
 .order_by（fieldname）
 .annotate（the_count = Count（fieldname））
  

有关此主题的以前的问题：

• < a href =https://stackoverflow.com/questions/629551/how-to-query-as-group-by-in-django>如何在django中查询为GROUP BY？


• 与GROUP BY相当于COUNT的Django

Lets say if I have a model that has lots of fields, but I only care about a charfield. Lets say that charfield can be anything so I don't know the possible values, but I know that the values frequently overlap. So I could have 20 objects with "abc" and 10 objects with "xyz" or I could have 50 objects with "def" and 80 with "stu" and i have 40000 with no overlap which I really don't care about.

How do I count the objects efficiently? What I would like returned is something like:

{'abc': 20, 'xyz':10, 'other': 10,000}

or something like that, w/o making a ton of SQL calls.

EDIT:

I dont know if anyone will see this since I am editing it kind of late, but...

I have this model:

class Action(models.Model):
    author = models.CharField(max_length=255)
    purl = models.CharField(max_length=255, null=True)

and from the answers, I have done this:

groups = Action.objects.filter(author='James').values('purl').annotate(count=Count('purl'))

but...

this is what groups is:

{"purl": "waka"},{"purl": "waka"},{"purl": "waka"},{"purl": "waka"},{"purl": "mora"},{"purl": "mora"},{"purl": "mora"},{"purl": "mora"},{"purl": "mora"},{"purl": "lora"}

(I just filled purl with dummy values)

what I want is

{'waka': 4, 'mora': 5, 'lora': 1}

Hopefully someone will see this edit...

EDIT 2:

Apparently my database (BigTable) does not support the aggregate functions of Django and this is why I have been having all the problems.

解决方案

You want something similar to "count ... group by". You can do this with the aggregation features of django's ORM:

from django.db.models import Count

fieldname = 'myCharField'
MyModel.objects.values(fieldname)
    .order_by(fieldname)
    .annotate(the_count=Count(fieldname))

Previous questions on this subject:

• How to query as GROUP BY in django?
• Django equivalent of COUNT with GROUP BY

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