在Django的Queryset中过滤django-taggit的标签 [英] filter tags of django-taggit in Django's Queryset

问题描述

拥有以下模型：

  class Post（models.Model）：
 title = models.CharField max_length = 250）
 tags = TaggableManager（）
  

，数据为：

  ** post.title ** ** post.tags ** 
 Django通过示例python，django，web 
谁是Django Reinhardt python，django，
使用Python进行测试驱动开发python，web 
数据分析Python Python数据
学习Python python 
编程Python python 
使用Python自动化无聊的东西python 
  

我尝试下面的代码

 >>> alist = Post.objects.filter（tags__name__in = [data，python]）
>>>因为我在alist.annotate（sam_tags = Count（'tags'））：
 ... print（\\\
 ---，i）
 ... print（ ：，i.tags.all（））
 ... print（sam_tags的值，i.sam_tags）
 ... 
 
 --- Django通过示例
标签：[<标签：django><标签：python><标签：web>] 
 sam_tags的值：1 
 
  -  - 谁是Django Reinhardt 
的标签：[<标签：django><标签：python>] 
 sam_tags的值：1 
 
 ---自动化Python的
标签：[<标签：python>] 
 sam_tags的值：$ 1 
 
 ---使用Python的测试驱动开发
标签：[<标签：python><标签：web>] 
 sam_tags的值：1 
 
 ---学习Python 
标签： [< Tag：python>] 
 sam_tags的值：1 
 
 ---用于数据分析的Python 
标签：[< Tag：data>标签：python>] 
 sam_tags的值：2 
 
 ---编程Python 
t它的标签：[<标签：python>] 
 sam_tags的值：1 
>>> 
  

为什么slist [0] .sam_tags（post：Django By Example）的值等于1

我认为（post：Django By Example）的post对象在阅读Django的文档后有三个标签[python，django和web]。

https：// docs。 djangoproject.com/en/1.10/ref/models/querysets/#count

它表示 Count（expression）返回通过提供的表达式相关的对象的数量。
所以代码

 >>> alist [0] .tags.all（）
 [<标签：django><标签：python><标签：web>] 
  

显示alist [0] .tags中有三个标签，

 >>> slist = alist.annotate（sam_tags = Count（'tags'））
>>> slist [0] .tags.all（）
 [< Tag：django><标签：python><标签：web>] 
>>> slist [0] .sam_tags 
 1 
  

但这里我得到的值1， >
为什么？

我知道Django只计算我的filter子句中包含的python和data标签 - 其他标签不是

slist [0] .tags.all（）的输出显示slist [0]有三个与自身相关的标签。因为django的文档表示Count（表达式）返回通过提供的表达式相关的对象的数量，slist [0] .sam_tags应该是3根据文档，但是django-taggit使slist [0] .sam_tags那么我真正想知道的是如何使用django-taggit让filter子句中的Count（表达式）只计算过滤条件中的标签数量。

解决方案

Django只计算 python 和数据您包含在过滤器子句中的标签 - 其他标记不计入。 （请注意， sam_tags 为2的唯一示例是标记为数据和 python 。）这可能是意想不到的行为，但如果您考虑如何执行底层SQL，这是有道理的。请参阅与您的类似模式的示例：

 >>> a = Article.objects.filter（tags__slug__in = ['python']）。annotate（num_tags = Count（'tags'））[0] 
>>> a.num_tags 
 1 
>>>> a.tags.count（）
 2 
  

如果我将filter子句更改为过滤除了标签之外，它的行为与预期的一样：

 >>> Article.objects.filter（pk = a.pk）.annotate（num_tags = Count（'tags'））[0] .num_tags 
 2 
>>> Article.objects.filter（pk = a.pk）.annotate（num_tags = Count（'tags'））[0] .tags.count（）
 2 
  / pre> 
Having the following models:
class Post(models.Model):
    title = models.CharField(max_length=250)
    tags = TaggableManager()
and the data are：
**post.title**                          **post.tags**
Django By Example                       python,django,web
Who was Django Reinhardt                python,django,
Test-Driven Development with Python     python,web
Python for Data Analysis                python,data         
Learning Python                         python
Programming Python                      python
Automate the Boring Stuff with Python   python
I try to code below
>>> alist=Post.objects.filter(tags__name__in=["data","python"])
>>> for i in alist.annotate(sam_tags=Count('tags')):
...     print("\n---",i)
...     print("tags of it : ",i.tags.all())
...     print("value of sam_tags : ",i.sam_tags)
... 

--- Django By Example
tags of it :  [<Tag: django>, <Tag: python>, <Tag: web>]
value of sam_tags :  1

--- Who was Django Reinhardt
tags of it :  [<Tag: django>, <Tag: python>]
value of sam_tags :  1

--- Automate the Boring Stuff with Python
tags of it :  [<Tag: python>]
value of sam_tags :  1

--- Test-Driven Development with Python
tags of it :  [<Tag: python>, <Tag: web>]
value of sam_tags :  1

--- Learning Python
tags of it :  [<Tag: python>]
value of sam_tags :  1

--- Python for Data Analysis
tags of it :  [<Tag: data>, <Tag: python>]
value of sam_tags :  2

--- Programming Python
tags of it :  [<Tag: python>]
value of sam_tags :  1
>>> 
why is the value of slist[0].sam_tags (post:Django By Example) equal to 1?

I think the post object of (post:Django By Example) has three tags [python,django and web] after reading the Django's documentation.

https://docs.djangoproject.com/en/1.10/ref/models/querysets/#count

It said that  Count(expression) returns the number of objects that are related through the provided expression. 
so the code 
>>>alist[0].tags.all() 
[<Tag: django>, <Tag: python>, <Tag: web>]
shows there are three tags in alist[0].tags,
>>> slist=alist.annotate(sam_tags=Count('tags'))
>>> slist[0].tags.all()
[<Tag: django>, <Tag: python>, <Tag: web>]
>>> slist[0].sam_tags
1
but here I get the value 1,

why?

I understand that Django is counting only the "python" and "data" tags which I included in my filter clause - other tags are not counted.

The output of "slist[0].tags.all()" shows that slist[0] has three tags related with itself. Because the documentation of django says that Count(expression) returns the number of objects that are related through the provided expression, slist[0].sam_tags should be 3 according to the documentation, but django-taggit make slist[0].sam_tags to be 1. 

So what I really want to know is how django-taggit lets Count(expression) in the filter clause only calculate the number of tags in the filter condition .
解决方案
Django is counting only the python and data tags which you included in your filter clause - other tags are not counted.  (Note that the only example with sam_tags of 2 is the one tagged both data and python.)  This is probably unexpected behavior, but makes sense if you consider how the underlying SQL is executed.  See this example from a similar schema to yours:
>>> a = Article.objects.filter(tags__slug__in=['python']).annotate(num_tags=Count('tags'))[0]
>>> a.num_tags
1
>>> a.tags.count()
2
If I change the filter clause to filter on somethign other than tags, it behaves as expected:
>>> Article.objects.filter(pk=a.pk).annotate(num_tags=Count('tags'))[0].num_tags
2
>>> Article.objects.filter(pk=a.pk).annotate(num_tags=Count('tags'))[0].tags.count()
2


                        
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