What's the difference between django OneToOneField and ForeignKey?

Question

What's the difference between Django OneToOneField and ForeignKey?

Answer 1

A ForeignKey is a many-to-one relationship. So, a Car object might have many instances of Wheel. Each Wheel would consequently have a ForeignKey to the Car it belongs to. A OneToOneField would be like an instance of Engine, where a Car object can have one and only one.

Answer 2

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 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 a QuerySet.

Example

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

1. Car model uses OneToOneField(Engine)
2. 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, on_delete=models.CASCADE)      def __unicode__(self):         return self.name