django - django OneToOneField 和 ForeignKey 有什么区别？

Question

Django OneToOneField 和 ForeignKey 有什么区别？

Answer 1

OneToOneField(SomeModel) 和 ForeignKey(SomeModel, unique=True) 之间的区别，如 The Definitive Guide to Django 中所述:

OneToOneField

A one-to-one relationship. Conceptually, this is similar to a ForeignKey with unique=True, but the "reverse" side of the relation will directly return a single object.

与OneToOneField“反向”关系相反，ForeignKey“反向”关系返回一个QuerySet。

例子

例如，如果我们有以下两个模型(下面是完整的模型代码):

1. Car 模型使用 OneToOneField(Engine)
2. Car2 模型使用 ForeignKey(Engine2, unique=True)

在 python manage.py shell 中执行以下命令:

OneToOneField 示例

>>> from testapp.models import Car, Engine
>>> c = Car.objects.get(name='Audi')
>>> e = Engine.objects.get(name='Diesel')
>>> e.car
<Car: Audi>

ForeignKey 与 unique=True 示例

>>> from testapp.models import Car2, Engine2
>>> c2 = Car2.objects.get(name='Mazda')
>>> e2 = Engine2.objects.get(name='Wankel')
>>> e2.car2_set.all()
[<Car2: Mazda>]

模型代码

from django.db import models

class Engine(models.Model):
    name = models.CharField(max_length=25)

    def __unicode__(self):
        return self.name

class Car(models.Model):
    name = models.CharField(max_length=25)
    engine = models.OneToOneField(Engine)

    def __unicode__(self):
        return self.name

class Engine2(models.Model):
    name = models.CharField(max_length=25)

    def __unicode__(self):
        return self.name

class Car2(models.Model):
    name = models.CharField(max_length=25)
    engine = models.ForeignKey(Engine2, unique=True, on_delete=models.CASCADE)

    def __unicode__(self):
        return self.name