继承模型上的Django ForeignKey _set [英] Django ForeignKey _set on an inherited model

问题描述

我有两个模型 Category 和 Entry 。还有另一个模型 ExtEntry 继承自 Entry

I have two models Category and Entry. There is another model ExtEntry that inherits from Entry

class Category(models.Model):
    title = models.CharField('title', max_length=255)
    description = models.TextField('description', blank=True)
    ...

class Entry(models.Model):
    title = models.CharField('title', max_length=255)    
    categories = models.ManyToManyField(Category)
    ...

class ExtEntry(Entry):    
    groups= models.CharField('title', max_length=255)
    value= models.CharField('title', max_length=255)
    ...

I能够使用 Category.entry_set ，但我希望能够使用 Category.blogentry_set 但它不可用。如果这不可用，那么我需要另一种方法来获取与一个特定类别

I am able to use the Category.entry_set but I want to be able to do Category.blogentry_set but it is not available. If this is not available,then I need another method to get all ExtEntryrelated to one particular Category

编辑
我的最终目标是拥有一个ExtEntry对象的QuerySet

EDIT My end goal is to have a QuerySet of ExtEntry objects

谢谢

推荐答案

我需要另一种方法来获取与某个特定类别相关的所有ExtEntry

I need another method to get all ExtEntryrelated to one particular Category

容易：

ExtEntry.objects.filter(categories=my_category)

---

---

您是否知道使用继承的_set功能的方法

Do you know if there is a way to use the _set feature of an inherited

我不知道它们是否有直接的用途。在文档中没有提到。

I don't know if there is a direct they for that. It is not mentioned in documentation.

但是使用 select_related 可以获得类似的结果。

But it is possible to get similar results with the select_related.

for e in category.entry_set.select_related('extentry'):
    e.extentry # already loaded because of `select_related`, 
               # however might be None if there is no Extentry for current e

仅选择具有ExtEntry的条目：

It is possible to select only entries which has ExtEntry:

for e in category.entry_set.select_related('extentry').exlude(extentry=None):
    e.extentry # now this definitely is something, not None

坏东西关于排除项的问题是，它会生成无效的查询：

Bad thing about the exclude is that it generates terrybly inefficient query:

SELECT entry.*, extentry.* FROM entry
LEFT OUTER JOIN `extentry` ON (entry.id = extentry.entry_ptr_id) 
WHERE NOT (entry.id IN (SELECT U0.id FROM entry U0 LEFT OUTER JOIN 
                        extentry U1 ON (U0.id = U1.entry_ptr_id) 
                        WHERE U1.entry_ptr_id IS NULL))

所以我的简历会是：使用 ExtEntry.objects.filter（）获得结果。向后关系（object.something_set）只是一种便利，并不适用于所有情况。

So my resume would be: use ExtEntry.objects.filter() to get your results. The backwards relations (object.something_set) is just a convenience and does not work in every situation.

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