What s the difference between django OneToOneField and ForeignKey

Question

What s the difference between Django OneToOneField and ForeignKey

User · Answer

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.

User · Answer

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

User · Answer

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

User · Answer

A ForeignKey is for one-to-many  so a Car object might have many Wheels  each Wheel having a ForeignKey to the Car it belongs to  A OneToOneField would be like an Engine  where a Car object can have one and only one

User · Answer

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

User · Answer

OneToOneField  Example  one car has one owner  ForeignKey OneToMany   Example  one restaurant has many items

User · Answer

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

User · Answer

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 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   gt  gt  gt  from testapp models import Car  Engine  gt  gt  gt  c   Car objects get name  Audi    gt  gt  gt  e   Engine objects get name  Diesel    gt  gt  gt  e car  lt Car  Audi gt    ForeignKey with unique True Example   gt  gt  gt  from testapp models import Car2  Engine2  gt  gt  gt  c2   Car2 objects get name  Mazda    gt  gt  gt  e2   Engine2 objects get name  Wankel    gt  gt  gt  e2 car2 set all     lt Car2  Mazda gt     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

User · Answer

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

User · Answer

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  amp  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     lt Article  Python News gt    lt Article  TDD In Django gt     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

[django] What's the difference between django OneToOneField and ForeignKey?

Examples related to django

Examples related to django-models