What's the difference between django OneToOneField and ForeignKey? - django

What's the difference between Django OneToOneField and ForeignKey?

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):
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, on_delete=models.CASCADE)
def __unicode__(self):
return self.name

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

Related

How to show the correct name object in Django admin instead 'XXX object'

I've two apps in my Django Project and I created an ManytoManyField relationship between then. However, when I check it on my admin site, the references appears showing the model name plus object inside the Many to Many box.
I already tried write a str in my model as you can see below:
def __str__(self):
return str(self.course.name)
Below there is my models code.
from django.db import models
from cursos.models import Course
class Person(models.Model):
name = models.CharField(max_length=50)
course = models.ManyToManyField(Course, blank=True)
def __str__(self):
return str(self.course.name)
This part of the form deals with editing the ManyToManyField. It will use the __str__ of the model that is referenced.
In order to thus change the textual representation, you need to implement the __str__ of the Course model, like:
class Course(models.Model): # Course model, not Person
name = models.CharField(max_length=128)
def __str__(self):
return self.name
Setting the __str__ of a person to self.course.name does not seem correct, since you probably do not want to represent a Person by the name of its courses. Furthermore since this is a ManyToManyField, it will not work anyway.
Note: usually the name of ManyToManyFields is plural, so courses, instead of course, since it is basically a collection of Courses, not a single Course.

r.article_set.all() does not make a query through .objects whereas returns a query

In Django's tutorial,Django at a glance | Django documentation | Django
r.article_set.all() does not make a query through .objects whereas returns a query.
It's quick example:
#mysite/news/models.py
from django.db import models
class Reporter(models.Model):
full_name = models.CharField(max_length=70)
def __str__(self): # __unicode__ on Python 2
return self.full_name
class Article(models.Model):
pub_date = models.DateField()
headline = models.CharField(max_length=200)
content = models.TextField()
reporter = models.ForeignKey(Reporter, on_delete=models.CASCADE)
def __str__(self): # __unicode__ on Python 2
return self.headline
Working in Django shell,
# Create a new Reporter.
>>> r = Reporter(full_name='John Smith')
# And vice versa: Reporter objects get API access to Article objects.
>>> r.article_set.all()
<QuerySet [<Article: Django is cool>]>
Return a QuerySet,even if a Managers is not called.
I search through the documentation and learn that A Manager is the interface through which database query operations are provided to Django models. At least one Manager exists for every model in a Django application.
So I can understand:
>>> Article.objects.all()
<QuerySet [<Article: Django is cool>]>
Django adds a Manager with the name objects to every Django model class. Managers | Django documentation | Django
As for r.article_set.all(), it does not make a query through .objects whereas returns a query.
article_set is neither the attribute of Reporter or django.db.models.Model
In [3]: hasattr('article_set', 'models.Model')
Out[3]: False
How does it work? or provide me a hint so I can explore further.
It goes through RelatedManager
A “related manager” is a manager used in a one-to-many or many-to-many
related context.
It is automatically added as part of each related field name so in your case Article has ForeignKey on Reporter which gives Reporter ability to get article_set same as to Article to get Reporter object (ex: article.reporter).
Additionally interesting info on how to do the custom reverse manager
Also don't be afraid to dig into Django source code to get better idea of how does it exactly work

Django model design: editable help text for individual model fields. Is there a foreign field that references a specific field of a model?

I have several models with several fields in my app. I want to set up a way for the user to be able to modify a help text system for each field in the model. Can you give me some guidance on how to design the models, and what field types to use? I don't feel right about storing the model and field name in CharFields, but if that is the only way, I may be stuck with it.
Is there a more elegant solution using Django?
For a quick and silly example, with an app named jobs, one named fun, and make a new app named helptext:
jobs.models.py:
class Person(models.Model):
first_name = models.CharField(max_length=32)
.
.
interests = models.TextField()
def __unicode__(self):
return self.name
class Job(models.Model):
name = models.CharField(max_length=128)
person = models.ForeignKey(Person)
address = models.TextField()
duties = models.TextField()
def __unicode__(self):
return self.name
fun.models.py:
class RollerCoaster(models.Model):
name = models.CharField(max_length=128)
scare_factor = models.PositiveInteger()
def __unicode__(self):
return self.name
class BigDipper(RollerCoaster):
max_elevation = models.PositiveInteger()
best_comment_ever_made = models.CharField(max_length=255)
def __unicode__(self):
return super.name
Now, let's say I want to have editable help text on Person.interests, and Job.duties, RollerCoaster.scare_factor, and BigDipper.best_comment_ever_made. I'd have something like:
helptext.models.py:
from django.contrib.contenttypes.models import ContentType
class HelpText(models.Model):
the_model = models.ForeignKey(ContentType)
the_field = models.CharField(max_length=255)
helptext = models.CharField(max_length=128)
def __unicode__(self):
return self.helptext
So, what is the better way to do this, other than making HelpText.the_model and HelpText.the_field CharFields that have to be compared when I am rendering the template to see if helptext is associated with each field on the screen?
Thanks in advance!
Edit:
I know about the help_text parameter of the fields, but I want this to be easily edited through the GUI, and it may contain a LOT of help with styling, etc. It would be HTML with probably upwards of 50-60 lines of text for probably 100 different model fields. I don't want to store it in the field definition for those reasons.
I changed the HelpText model to have a reference to ContentType and the field a CharField. Does this seem like a good solution? I am not sure this is the most elegant way. Please advise.
Edit 2013-04-19 16:53 PST:
Currently, I tried this and it works, but not sure this is great:
from django.db import models
from django.contrib.contenttypes.models import ContentType
# Field choices for the drop down.
FIELDS = ()
# For each ContentType verify the model_class() is not None and if not, add a tuple
# to FIELDS with the model name and field name displayed, but storing only the field
# name.
for ct in ContentType.objects.all():
m = ct.model_class()
if m is not None:
for f in ct.model_class()._meta.get_all_field_names():
FIELDS += ((f, str(ct.model) + '.' + str(f)),)
# HelpText model, associated with multiple models and fields.
class HelpText(models.Model):
the_model = models.ForeignKey(ContentType)
the_field = models.CharField(max_length=255, choices=FIELDS)
helptext = models.TextField(null=True, blank=True)
def __unicode__(self):
return self.helptext
Doesn't feel like the best, but please advise if this is a solution that will bite me in the behind later on and make me filled with regrets... :*(
The solution works, and I have it implemented, but you have to be aware that sometimes the ContentTypes get out of sync with your models. You can manually update the content types with this:
python manage.py shell
>>> from django.contrib.contenttypes.management import update_all_contenttypes
>>> update_all_contenttypes(interactive=True)
This allows you to add the new ones and remove the old ones, if they exist.
The nice thing about the Field not being a foreign key is that I can put anything in it for help text. So, say I have a field "First Name." I can put a helptext connected to the Person model and the "first_name" field. I can also make something up, like "Something really confusing." The helptext is now associated with the Person model and the "Something really confusing" field. So, I can put it at the top of the form, instead of associating to a field with hard foreign keying. It can be anything arbitrary and will follow with that "field" anywhere. The hangup would be that you may change the name of the helptext field association inadvertently sending your original helptext into never land.
To make this easy, I created a TemplateTag, which I pass the name of the model and the name of the "field" I want to associate. Then anytime the template is rendered, that helptext is there, editable for anybody to get assistance with their user interface forms.
Not sure this is the best solution, but I couldn't really see any other way to do it, and got no responses.
Cheerio!

Difference between ForeignKey(User, unique=True) and OneToOneField [duplicate]

This question already has answers here:
What's the difference between django OneToOneField and ForeignKey?
(11 answers)
Closed 8 years ago.
What is different between models.ForeignKey(Modelname, unique=True) and models.OneToOneField in Django?
Where should I use models.OneToOneField and models.ForeignKey(Modelname, unique=True)?
A OneToOneField is very similar to a ForeignKey with unique=True. Unless you are doing multiple table inheritance, in which case you have to use OneToOneField, the only real difference is the api for accessing related objects.
In the Django docs it says:
Conceptually, this is similar to a ForeignKey with unique=True, but the "reverse" side of the relation will directly return a single object.
Let's show what that means with an example. Consider two models, Person and Address. We'll assume each person has a unique address.
class Person(models.Model):
name = models.CharField(max_length=50)
address = models.ForeignKey('Address', unique=True)
class Address(models.Model):
street = models.CharField(max_length=50)
If you start with a person, you can access the address easily:
address = person.address
However if you start with an address, you have to go via the person_set manager to get the person.
person = address.person_set.get() # may raise Person.DoesNotExist
Now let's replace the ForeignKey with a OneToOneField.
class Person(models.Model):
name = models.CharField(max_length=50)
address = models.OneToOneField('Address')
class Address(models.Model):
street = models.CharField(max_length=50)
If you start with a person, you can access the address in the same way:
address = person.address
And now, we can access the person from the address more easily.
person = address.person # may raise Person.DoesNotExist
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 hope this helps illustrate the differences between the relationships created.

ForeignKey to multiple Models or Queryset

It is possible to make a ForeignKey to more than one model. I want to choose from different models like Parts and Machines Model.
I read this to combine multiple models into one list: How to combine 2 or more querysets in a Django view?
How can I get foreign key to that list somehow?
I know that you asked this over year ago, but I had a similar problem and I want to share a link to the solution for future readers.
Generally the contenttypes framework solves this problem, and I guess this is what Daniel Roseman was talking about.
How to use dynamic foreignkey in Django?
You need generic relations.
A generic relation allows you to dynamically the target model of the foreign key.
I'll provide a comprehensive answer for this question, I know its quite old, but it's still relevant.
We're gonna be using Generic Relations.
First, in settings.py make sure that django.contrib.contenttypes is included in the INSTALLED_APPS array.
Let's create a new model in models.py:
from django.contrib.contenttypes.models import ContentType
from django.contrib.contenttypes.fields import GenericForeignKey, GenericRelation
With content_type we can associate Image with any other model class, while object_id will hold the other model instance.
class Image(models.Model):
image = models.ImageField(
upload_to="imgs/products", blank=True)
content_type = models.ForeignKey(ContentType, on_delete=models.CASCADE)
object_id = models.PositiveIntegerField()
content_object = GenericForeignKey()
To refer back to the Image model from a Company instance we need to make a reverse generic relation
class Company(models.Model):
name = models.CharField(max_length=100)
images = GenericRelation(Image)
In schema.py, we can create Images in a Company instance like:
company_instance = Company(name="Apple")
company_instance.save()
for img in imgs:
#Image(image=img, content_object=company_instance)
company_instance.images.create(image=img)
company_instance.images.all() # fetch all images
the company_instance.images field is just a GenericRelatedObjectManager (docs)
This is how the final Image table looks in the database:
The Django-polymorphic library provides a simple solution that should work well with the admin and forms too using formsets.
For example:
from polymorphic.models import PolymorphicModel
class BookOwner(PolymorphicModel):
book = models.ForeignKey(Book, on_delete=models.CASCADE)
class StaffBookOwner(BookOwner):
owner = models.ForeignKey(Staff, on_delete=models.CASCADE)
class StudentBookOwner(BookOwner):
owner = models.ForeignKey(Student, on_delete=models.CASCADE)
With this, you can use the parent model to set the owner to either a Staff or Student instance or use the child models directly.