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*）。

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