difference between foreign key and one to one field

OneToOneField (one-to-one) realizes, in object orientation, the notion of composition, while ForeignKey (one-to-many) relates to agregation.

Be careful to realize that there are some differences between OneToOneField(SomeModel) and ForeignKey(SomeModel, unique=True).

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.

(one-to-one field )(one-to-one)== (ForeignKey with unique=True)(one-to-many)

 ForeignKey ="reverse" relation returns a QuerySet.

one-to-one field = reverse" side of the relation will directly return a single object.

MODEL 

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)

    def __unicode__(self):
        return self.name

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) means Engine2 can have many Car2 instance queryset

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>]   #this is queryset

Another Explaination:

The best and the most effective way to learn new things is to see and study real world practical examples. Suppose for a moment that you want to build a blog in django where reporters can write and publish news articles. The owner of the online newspaper wants to allow each of his reporters to publish as many articles as they want, but does not want different reporters to work on the same article. This means that when readers go and read an article they will se only one author in the article.

For example: Article by John, Article by Harry, Article by Rick. You can not have Article by Harry & Rick because the boss does not want two or more authors to work on the same article.

How can we solve this 'problem' with the help of django? The key to the solution of this problem is the django ForeignKey.

The following is the full code which can be used to implement the idea of our boss.

from django.db import models

# Create your models here.

class Reporter(models.Model):
    first_name = models.CharField(max_length=30)

    def __unicode__(self):
        return self.first_name


class Article(models.Model):
    title = models.CharField(max_length=100)
    reporter = models.ForeignKey(Reporter)

    def __unicode__(self):
        return self.title

Run python manage.py syncdb to execute the sql code and build the tables for your app in your database. Then use python manage.py shell to open a python shell.

Create the Reporter object R1.

In [49]: from thepub.models import Reporter, Article

In [50]: R1 = Reporter(first_name='Rick')

In [51]: R1.save()

Create the Article object A1.

In [5]: A1 = Article.objects.create(title='TDD In Django', reporter=R1)

In [6]: A1.save()

Then use the following piece of code to get the name of the reporter.

In [8]: A1.reporter.first_name
Out[8]: 'Rick'

Now create the Reporter object R2 by running the following python code.

In [9]: R2 = Reporter.objects.create(first_name='Harry')

In [10]: R2.save()

Now try to add R2 to the Article object A1.

In [13]: A1.reporter.add(R2)

It does not work and you will get an AttributeError saying 'Reporter' object has no attribute 'add'.

As you can see an Article object can not be related to more than one Reporter object.

What about R1? Can we attach more than one Article objects to it?

In [14]: A2 = Article.objects.create(title='Python News', reporter=R1)

In [15]: R1.article_set.all()
Out[15]: [<Article: Python News>, <Article: TDD In Django>]

This practical example shows us that django ForeignKey is used to define many-to-one relationships.

OneToOneField is used to create one-to-one relationships.

We can use reporter = models.OneToOneField(Reporter) in the above models.py file but it is not going to be useful in our example as an author will not be able to post more than one article.

Each time you want to post a new article you will have to create a new Reporter object. This is time consuming, isn't it?

I highly recommend to try the example with the OneToOneField and realize the difference. I am pretty sure that after this example you will completly know the difference between django OneToOneField and django ForeignKey.