What's the difference between Django OneToOneField
and ForeignKey
?
Be careful to realize that there are some differences between OneToOneField(SomeModel)
and ForeignKey(SomeModel, unique=True)
. As stated in The Definitive Guide to Django:
OneToOneField
A one-to-one relationship. Conceptually, this is similar to a
ForeignKey
withunique=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 a QuerySet
.
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>]
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