Django模型中的OneToOneField和ForeignKey有什么区别？

说是ForeignKey是one-to-many的，并举了一个车的例子：

有两个配件表，一个是车轮表，另一个是引擎表。两个表都有一个car字段，表示该配件对应的车。
对于车轮来说，多个对应一个car的情况很正常，所以car字段应该用ForeignKey来表示。
对于引擎来说，一个引擎只可能对应一个car，所以必须用OneToOneField。

OneToOneField(someModel) 可以理解为 ForeignKey(SomeModel, unique=True)。

两者的反向查询是有差别的：

ForeignKey反向查询返回的是一个列表（一个车有多个轮子）。

OneToOneField反向查询返回的是一个模型示例（因为一对一关系）。

另外的补充说明：

Be careful to realize that there are some differences between OneToOneField(SomeModel) andForeignKey(SomeModel, unique=True). As stated in The Definitive Guide to Django:

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.

In contrast to the OneToOneField "reverse" relation, a ForeignKey "reverse" relation returns aQuerySet.

Example
For example, if we have the following two models (full model code below):

Car model uses OneToOneField(Engine)
Car2 model uses ForeignKey(Engine2, unique=True)
From within python manage.py shell execute the following:

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

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

Model Code

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)

引自redice,侵删