多表继承模型和相同的两个模型之间的简单一对一关系有什么区别? [英] What is the difference between a Multi-table inherited model and a simple One-to-one relationship between the same two models?

问题描述

这些实现之间有什么区别?Django 有什么不同(除了继承 Meta ordering 和 get_latest_by 属性)?

What is the differences between these implementations? What does Django differently (beside inheriting Meta ordering and get_latest_by attribute) ?

# models.py
from django.db import models

class Place(models.Model):
    name = models.CharField(max_length=50)

class Restaurant(models.Model):
    place = models.OneToOneField(Place)
    serves_pizza = models.BooleanField()

2.

class Place(models.Model):
    name = models.CharField(max_length=50)

class Restaurant(Place):
    serves_pizza = models.BooleanField()

3.

class Place(models.Model):
    name = models.CharField(max_length=50)

class Restaurant(Place):
    place = models.OneToOneField(Place, parent_link=True)
    serves_pizza = models.BooleanField()

推荐答案

1. 你并没有真正获得任何 Python 继承，也就是说你不能从模型类 Place 在你的类 Restaurant 中:

1. You don't really get any python inheritance, that is you can't inherit/override methods or attributes from the model class Place in your class Restaurant:

例如:

class Place(models.Model):
    name = models.CharField(max_length=50)

    def get_x(self):
        return 'x'

class Restaurant(models.Model):
    place = models.OneToOneField(Place)
    serves_pizza = models.BooleanField()

a_restaurant = Restaurant()
a_restaurant.get_x() # -> wouldn't work

这意味着要获取餐厅的name，您无法使用a_restaurant.name，您需要点击以下链接:a_restaurant.place.name

This means that to obtain the name of a restaurant you can't do a_restaurant.name, you would need to follow the link: a_restaurant.place.name

另请注意，在查询具有相关 Restaurant 的 Place 对象时.

Also note that when querying a Place object with the related Restaurant.

a_restaurant.save()
Place.objects.get(pk=a_restaurant.pk)  # won't work

你必须写:

a_restaurant.save()
Place.objects.get(restaurant__pk=a_restaurant.pk)

<小时>

2 和 3.几乎相同.你确实得到了真正的 Python 继承.

---

2 and 3. are almost the same. You do get real python inheritance with these.

a_restaurant = Restaurant()
a_restaurant.get_x() # would actually work and print 'x'

您的模型类 Restaurant 继承了 Place 的所有内容:模型字段、普通实例/类属性、管理器、方法……您还可以覆盖其中的几乎任何内容:您不能覆盖字段属性，这是不受支持的.

Your model class Restaurant inherits everything from Place: model fields, normal instance/class attributes, managers, methods... and you can also override almost anything of these: You can't override field attributes, that's not supported.

所以现在您可以直接从父模型中获取字段的值:a_restaurant.name 因为它们是继承的.

So now you can get the values of the fields from the parent model directly:a_restaurant.name because they are inherited.

由于有了这些实现，Restaurant is 也是 Place，您可以使用 Place 查询Place 对象代码>餐厅数据:

Since with these implementation a Restaurant is a also Place you can query for a Place object with Restaurant data:

a_restaurant.save()
the_place = Place.objects.get(pk=a_restaurant.pk)  
# ^ this works now and returns the equivalent `Place` instance.
the_same_restaurant = the_place.restaurant

<小时>

如果您为该字段指定不同的名称，则更容易看出 2 和 3 之间的区别:

class Place(models.Model):
    name = models.CharField(max_length=50)

class Restaurant(Place):
    where = models.OneToOneField(Place, parent_link=True)
    serves_pizza = models.BooleanField()

工作原理完全相同，但要获取Restaurant 的父位置，属性名称为where:

Works exactly the same but to obtain the parent place of a Restaurant the attribute name is where:

the_place = a_restaurant.where

使用 2 本来是:

the_place = a_restaurant.place_ptr

这意味着 place = models.OneToOneField(Place, parent_link=True) 只会更改父模型实例的链接名称.默认名称为 '{lowercase_model_name}_ptr'.

These means that place = models.OneToOneField(Place, parent_link=True) will only change the name of the link to the parent model instance. The default name is '{lowercase_model_name}_ptr'.

最后一个例子:

使用1:

place1 = Place.objects.create(name='place_1')
place2 = Place.objects.create(name='place_2')
restaurant1 = Restaurant.objects.create(place=place1, serves_pizza=True)

print Place.objects.all() # prints [place1, place2]
print Restaurant.objects.all() # prints [restaurant1]

与2-3:

place1 = Place.objects.create(name='place_1')
place2 = Place.objects.create(name='place_2')
restaurant1 = Restaurant.objects.create(name='place_3', serves_pizza=True)

print Place.objects.all() # prints [place1, place2, place3]
print Restaurant.objects.all() # prints [restaurant1]

<小时>

希望这些有帮助.它长得太长了:/

---

Hope these helps. It grow a bit too long :/

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