[Django]-OneToOneField() vs ForeignKey() in Django

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 has at most one and only one.

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.

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

Also OneToOneField is useful to be used as primary key to avoid key duplication. One may do not have implicit / explicit autofield

models.AutoField(primary_key=True)

but use OneToOneField as primary key instead (imagine UserProfile model for example):

user = models.OneToOneField(
    User, null=False, primary_key=True, verbose_name='Member profile')

When you access a OneToOneField you get the value of the field you queried. In this example a book model’s ‘title’ field is a OneToOneField:

>>> from mysite.books.models import Book
>>> b = Book.objects.get(id=50)
>>> b.title
u'The Django Book'

When you access a ForeignKey you get the related model object, which you can then preform further queries against. In this example the same book model’s ‘publisher’ field is a ForeignKey (correlating to the Publisher class model definition):

>>> b = Book.objects.get(id=50)
>>> b.publisher
<Publisher: Apress Publishing>
>>> b.publisher.website
u'http://www.apress.com/'

With ForeignKey fields queries work the other way too, but they’re slightly different due to the non-symmetrical nature of the relationship.

>>> p = Publisher.objects.get(name='Apress Publishing')
>>> p.book_set.all()
[<Book: The Django Book>, <Book: Dive Into Python>, ...]

Behind the scenes, book_set is just a QuerySet and can be filtered and sliced like any other QuerySet. The attribute name book_set is generated by appending the lower case model name to _set.

I have also been confused with the usage of both the fields.
Let me give an example for understanding their usage, as I have faced the problem recently and realised the usage of both the fields.

I had a model, like this-

from django.contrib.auth.models import User
from django.db import models


class Attendance(models.Model):
     user = models.OneToOneField(User, on_delete=models.CASCADE, default="", null=True)
     date = models.CharField(max_length=11)

     def __int__(self):
         return self.id

Now the problem with this was that I can’t make multiple objects with the same user,
i.e. a same user will have attendance on multiple days. Hence, multiple objects with same user.

But the OneToOne field didn’t let me do that.
Image for reference

So, I changed my model to-

from django.contrib.auth.models import User
from django.db import models


class Attendance(models.Model):
    user = models.ForeignKey(User, on_delete=models.CASCADE, default="", null=True)
    date = models.CharField(max_length=11)

    def __int__(self):
        return self.id

Now it works fine and I can mark attendance for a user on multiple days.

So that’s where the difference is, OneToOne field will not allow you to make multiple objects with the same user(as an example) but with ForeignKey it is possible.

OneToOneField: if second table is related with

table2_col1 = models.OneToOneField(table1,on_delete=models.CASCADE, related_name='table1_id')

table2 will contains only one record corresponding to table1’s pk value, i.e table2_col1 will have unique value equal to pk of table

table2_col1 == models.ForeignKey(table1, on_delete=models.CASCADE, related_name='table1_id')

table2 may contains more than one record corresponding to table1’s pk value.

The easiest way to draw a relationship between items is by understanding them in plain languages. Example

A user can have many cars but then a car can have just one owner. After establishing this, the foreign key should be used on the item with the many relationship. In this case the car. Meaning you’ll include user as a foreign key in cars

And a one on one relationship is quite simple. Say a man and a heart. A man has only one heart and a heart can belong to just one man

OneToOneField (Example: one car has one owner)
ForeignKey(OneToMany) (Example: one restaurant has many items)

ForeignKey allows you receive subclasses is it definition of another class but OneToOneFields cannot do this and it is not attachable to multiple variables

OneToOneField() cannot access the child model with _set while ForeignKey() and ManyToManyField() can. *You can see my question and the answer about it.

For example, you have Person model and PersonDetail model with OneToOneField() as shown below:

class Person(models.Model):
    name = models.CharField(max_length=20)

class PersonDetail(models.Model):
    person = models.OneToOneField(Person, on_delete=models.CASCADE)
    age = models.IntegerField()
    gender = models.CharField(max_length=20)

Then, you cannot access the child model PersonDetail with persondetail_set of a Person object as shown below because there is error. *Use persondetail instead of persondetail_set to access the child model PersonDetail in this case of OneToOneField():

obj = Person.objects.get(id=1)
print(obj.persondetail_set.get(id=1)) # Error
        # ↑ ↑ ↑ Here ↑ ↑ ↑

Now, you use ForeignKey() and ManyToManyField() as shown below:

class PersonDetail(models.Model):
    person = models.ForeignKey(Person, on_delete=models.CASCADE)
    # ...

class PersonDetail(models.Model):
    person = models.ManyToManyField(Person)
    # ...

Then, you can access the child model PersonDetail with persondetail_set of a Person object as shown below:

obj = Person.objects.get(id=1)
print(obj.persondetail_set.get(id=1)) # PersonDetail object (1)
        # ↑ ↑ ↑ Here ↑ ↑ ↑