Django-taggit prefetch_related [英] Django-taggit prefetch_related

问题描述

我正在构建一个基本的时间记录应用程序，我有一个使用django-taggit的todo模型。我的Todo模型看起来像这样：

 类Todo（models.Model）：
 project = models.ForeignKey（Project ）
 description = models.CharField（max_length = 300）
 is_done = models.BooleanField（default = False）
 billable = models.BooleanField（default = True）
 date_completed = models .DateTimeField（blank = True，null = True）
 completed_by = models.ForeignKey（User，blank = True，null = True）
 tags = TaggableManager（）
 
 def __unicode__ （自）：
 return self.description 
  

我正在尝试获取一份项目中所有Todos的独特标签，我已经设法让这个工作使用集合的理解，但是对于项目中的每个Todo，我必须查询数据库以获取标签。我的设置理解是：在project.todo_set.all（）中为

  unique_tags = {tag.name.lower（）fordodo标签在todo.tags.all（）} 
  

这个工作很好，但是对于每一个项目运行一个单独的查询来抓取所有的标签。我想知道是否有任何方式可以做类似于prefetch_related的事情，以避免这些重复的查询：

  unique_tags = { tag.name.lower（）for project.todo_set.all（）。prefetch_related（'tags'）forto.tags.all（）} 
  

运行以前的代码给我错误：

 'tags'不解决支持预取的项目 - 这是prefetch_related（）的无效参数。 
  

我在这里看到有人问了一个非常类似的问题：优化django查询以拉取外键和django-taggit关系，但是它没有看起来好像有明确的答案。我希望有人可以帮助我。谢谢！

解决方案

Taggit现在支持 prefetch_related 直接在标签字段版本0.11.0及更高版本，2013-11-25发布）。

此功能在此拉请求。在测试用例中，请注意，在预取后标签使用 .prefetch_related（'tags'），还有0个额外的查询列表标签。

I'm building a basic time logging app right now and I have a todo model that uses django-taggit. My Todo model looks like this:

class Todo(models.Model):
    project = models.ForeignKey(Project)
    description = models.CharField(max_length=300)
    is_done = models.BooleanField(default=False)
    billable = models.BooleanField(default=True)
    date_completed = models.DateTimeField(blank=True, null=True)
    completed_by = models.ForeignKey(User, blank=True, null=True)
    tags = TaggableManager()

    def __unicode__(self):
        return self.description

I'm trying to get a list of unique tags for all the Todos in a project and I have managed to get this to work using a set comprehension, however for every Todo in the project I have to query the database to get the tags. My set comprehension is:

unique_tags = { tag.name.lower() for todo in project.todo_set.all() for tag in todo.tags.all() }

This works just fine, however for every todo in the project it runs a separate query to grab all the tags. I was wondering if there is any way I can do something similar to prefetch_related in order to avoid these duplicate queries:

unique_tags = { tag.name.lower() for todo in project.todo_set.all().prefetch_related('tags') for tag in todo.tags.all() }

Running the previous code gives me the error:

'tags' does not resolve to a item that supports prefetching - this is an invalid parameter to prefetch_related().

I did see that someone asked a very similar question here: Optimize django query to pull foreign key and django-taggit relationship however it doesn't look like it ever got a definite answer. I was hoping someone could help me out. Thanks!

解决方案

Taggit now supports prefetch_related directly on tag fields (in version 0.11.0 and later, released 2013-11-25).

This feature was introduced in this pull request. In the test case for it, notice that after prefetching tags using .prefetch_related('tags'), there are 0 additional queries for listing the tags.

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