在单个查询中获取外键对象 - Django [英] get foreign key objects in a single query - Django

问题描述

我的django代码中有两个模型：

  class ModelA（models.Model）：
 name = models.CharField（max_length = 255）
 description = models.CharField（max_length = 255）
 created_by = models.ForeignKey（User）
 
 class ModelB（models.Model） 
 category = models.CharField（max_length = 255）
 modela_link = models.ForeignKey（ModelA，'modelb_link'）
 functions = models.CharField（max_length = 255）
 created_by =模型.ForeignKey（用户）
  

说ModelA有100条记录，所有这些记录可能有也可能没有链接到ModelB

现在说我想得到每个ModelA记录的列表以及来自ModelB的数据

我会这样做：

  list_a = ModelA.objects.all（）
  

然后为了获取ModelB的数据，我将不得不做

 对于我在list_a：
 i.additional_data = i.modelb_link.all（）
  

但是，这会对i的每个实例运行一个查询。从而使101个查询运行。

有没有任何方法可以在一个查询中运行这一切。或至少少于101个查询。

我尝试放入 ModelA.objects.select_related（）。all（）但这没有似乎有任何效果。

谢谢

解决方案

As Ofri说， select_related 只适用于前向关系，而不是反向关系。

没有内置的方法来自动跟踪Django中的反向关系，但请参阅我的博客文章，以便有效地做到这一点。基本思想是一次获取每个项目的所有相关对象，然后将它们与他们的相关项目手工关联 - 所以您可以在2个查询而不是n + 1中执行此操作。

I have 2 models in my django code:

class ModelA(models.Model):
    name = models.CharField(max_length=255)
    description = models.CharField(max_length=255)
    created_by = models.ForeignKey(User)

class ModelB(models.Model):
    category = models.CharField(max_length=255)
    modela_link = models.ForeignKey(ModelA, 'modelb_link')
    functions = models.CharField(max_length=255)
    created_by = models.ForeignKey(User)

Say ModelA has 100 records, all of which may or may not have links to ModelB

Now say I want to get a list of every ModelA record along with the data from ModelB

I would do:

list_a = ModelA.objects.all()

Then to get the data for ModelB I would have to do

for i in list_a:
    i.additional_data = i.modelb_link.all()

However this runs a query on every instance of i. Thus making 101 queries to run.

Is there any way of running this all in just 1 query. Or at least less than the 101 queries.

I've tried putting in ModelA.objects.select_related().all() but this didn't seem to have any effect.

Thanks

解决方案

As Ofri says, select_related only works on forwards relations, not reverse ones.

There's no built-in way to automatically follow reverse relations in Django, but see my blog post for a technique to do it reasonably efficiently. The basic idea is to get all the related objects for every item at once, then associate them manually with their related item - so you can do it in 2 queries rather than n+1.

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