To define a many-to-many relationship, use ManyToManyField.
In this example, an Article can be published in multiple Publication objects, and a Publication has multiple Article objects:
from django.db import models
class Publication(models.Model):
title = models.CharField(max_length=30)
def __unicode__(self):
return self.title
class Meta:
ordering = ('title',)
class Article(models.Model):
headline = models.CharField(max_length=100)
publications = models.ManyToManyField(Publication)
def __unicode__(self):
return self.headline
class Meta:
ordering = ('headline',)
What follows are examples of operations that can be performed using the Python API facilities.
Create a couple of Publications:
>>> p1 = Publication(title='The Python Journal')
>>> p1.save()
>>> p2 = Publication(title='Science News')
>>> p2.save()
>>> p3 = Publication(title='Science Weekly')
>>> p3.save()
Create an Article:
>>> a1 = Article(headline='Django lets you build Web apps easily')
You can’t associate it with a Publication until it’s been saved:
>>> a1.publications.add(p1)
Traceback (most recent call last):
...
ValueError: 'Article' instance needs to have a primary key value before a many-to-many relationship can be used.
Save it!
>>> a1.save()
Associate the Article with a Publication:
>>> a1.publications.add(p1)
Create another Article, and set it to appear in both Publications:
>>> a2 = Article(headline='NASA uses Python')
>>> a2.save()
>>> a2.publications.add(p1, p2)
>>> a2.publications.add(p3)
Adding a second time is OK:
>>> a2.publications.add(p3)
Adding an object of the wrong type raises TypeError:
>>> a2.publications.add(a1)
Traceback (most recent call last):
...
TypeError: 'Publication' instance expected
Add a Publication directly via publications.add by using keyword arguments:
>>> new_publication = a2.publications.create(title='Highlights for Children')
Article objects have access to their related Publication objects:
>>> a1.publications.all()
[<Publication: The Python Journal>]
>>> a2.publications.all()
[<Publication: Highlights for Children>, <Publication: Science News>, <Publication: Science Weekly>, <Publication: The Python Journal>]
Publication objects have access to their related Article objects:
>>> p2.article_set.all()
[<Article: NASA uses Python>]
>>> p1.article_set.all()
[<Article: Django lets you build Web apps easily>, <Article: NASA uses Python>]
>>> Publication.objects.get(id=4).article_set.all()
[<Article: NASA uses Python>]
Many-to-many relationships can be queried using lookups across relationships:
>>> Article.objects.filter(publications__id__exact=1)
[<Article: Django lets you build Web apps easily>, <Article: NASA uses Python>]
>>> Article.objects.filter(publications__pk=1)
[<Article: Django lets you build Web apps easily>, <Article: NASA uses Python>]
>>> Article.objects.filter(publications=1)
[<Article: Django lets you build Web apps easily>, <Article: NASA uses Python>]
>>> Article.objects.filter(publications=p1)
[<Article: Django lets you build Web apps easily>, <Article: NASA uses Python>]
>>> Article.objects.filter(publications__title__startswith="Science")
[<Article: NASA uses Python>, <Article: NASA uses Python>]
>>> Article.objects.filter(publications__title__startswith="Science").distinct()
[<Article: NASA uses Python>]
The count() function respects distinct() as well:
>>> Article.objects.filter(publications__title__startswith="Science").count()
2
>>> Article.objects.filter(publications__title__startswith="Science").distinct().count()
1
>>> Article.objects.filter(publications__in=[1,2]).distinct()
[<Article: Django lets you build Web apps easily>, <Article: NASA uses Python>]
>>> Article.objects.filter(publications__in=[p1,p2]).distinct()
[<Article: Django lets you build Web apps easily>, <Article: NASA uses Python>]
Reverse m2m queries are supported (i.e., starting at the table that doesn’t have a ManyToManyField):
>>> Publication.objects.filter(id__exact=1)
[<Publication: The Python Journal>]
>>> Publication.objects.filter(pk=1)
[<Publication: The Python Journal>]
>>> Publication.objects.filter(article__headline__startswith="NASA")
[<Publication: Highlights for Children>, <Publication: Science News>, <Publication: Science Weekly>, <Publication: The Python Journal>]
>>> Publication.objects.filter(article__id__exact=1)
[<Publication: The Python Journal>]
>>> Publication.objects.filter(article__pk=1)
[<Publication: The Python Journal>]
>>> Publication.objects.filter(article=1)
[<Publication: The Python Journal>]
>>> Publication.objects.filter(article=a1)
[<Publication: The Python Journal>]
>>> Publication.objects.filter(article__in=[1,2]).distinct()
[<Publication: Highlights for Children>, <Publication: Science News>, <Publication: Science Weekly>, <Publication: The Python Journal>]
>>> Publication.objects.filter(article__in=[a1,a2]).distinct()
[<Publication: Highlights for Children>, <Publication: Science News>, <Publication: Science Weekly>, <Publication: The Python Journal>]
Excluding a related item works as you would expect, too (although the SQL involved is a little complex):
>>> Article.objects.exclude(publications=p2)
[<Article: Django lets you build Web apps easily>]
If we delete a Publication, its Articles won’t be able to access it:
>>> p1.delete()
>>> Publication.objects.all()
[<Publication: Highlights for Children>, <Publication: Science News>, <Publication: Science Weekly>]
>>> a1 = Article.objects.get(pk=1)
>>> a1.publications.all()
[]
If we delete an Article, its Publications won’t be able to access it:
>>> a2.delete()
>>> Article.objects.all()
[<Article: Django lets you build Web apps easily>]
>>> p2.article_set.all()
[]
Adding via the ‘other’ end of an m2m:
>>> a4 = Article(headline='NASA finds intelligent life on Earth')
>>> a4.save()
>>> p2.article_set.add(a4)
>>> p2.article_set.all()
[<Article: NASA finds intelligent life on Earth>]
>>> a4.publications.all()
[<Publication: Science News>]
Adding via the other end using keywords:
>>> new_article = p2.article_set.create(headline='Oxygen-free diet works wonders')
>>> p2.article_set.all()
[<Article: NASA finds intelligent life on Earth>, <Article: Oxygen-free diet works wonders>]
>>> a5 = p2.article_set.all()[1]
>>> a5.publications.all()
[<Publication: Science News>]
Removing publication from an article:
>>> a4.publications.remove(p2)
>>> p2.article_set.all()
[<Article: Oxygen-free diet works wonders>]
>>> a4.publications.all()
[]
And from the other end:
>>> p2.article_set.remove(a5)
>>> p2.article_set.all()
[]
>>> a5.publications.all()
[]
Relation sets can be assigned. Assignment clears any existing set members:
>>> a4.publications.all()
[<Publication: Science News>]
>>> a4.publications = [p3]
>>> a4.publications.all()
[<Publication: Science Weekly>]
Relation sets can be cleared:
>>> p2.article_set.clear()
>>> p2.article_set.all()
[]
And you can clear from the other end:
>>> p2.article_set.add(a4, a5)
>>> p2.article_set.all()
[<Article: NASA finds intelligent life on Earth>, <Article: Oxygen-free diet works wonders>]
>>> a4.publications.all()
[<Publication: Science News>, <Publication: Science Weekly>]
>>> a4.publications.clear()
>>> a4.publications.all()
[]
>>> p2.article_set.all()
[<Article: Oxygen-free diet works wonders>]
Recreate the article and Publication we have deleted:
>>> p1 = Publication(title='The Python Journal')
>>> p1.save()
>>> a2 = Article(headline='NASA uses Python')
>>> a2.save()
>>> a2.publications.add(p1, p2, p3)
Bulk delete some Publications - references to deleted publications should go:
>>> Publication.objects.filter(title__startswith='Science').delete()
>>> Publication.objects.all()
[<Publication: Highlights for Children>, <Publication: The Python Journal>]
>>> Article.objects.all()
[<Article: Django lets you build Web apps easily>, <Article: NASA finds intelligent life on Earth>, <Article: NASA uses Python>, <Article: Oxygen-free diet works wonders>]
>>> a2.publications.all()
[<Publication: The Python Journal>]
Bulk delete some articles - references to deleted objects should go:
>>> q = Article.objects.filter(headline__startswith='Django')
>>> print(q)
[<Article: Django lets you build Web apps easily>]
>>> q.delete()
After the delete, the QuerySet cache needs to be cleared, and the referenced objects should be gone:
>>> print(q)
[]
>>> p1.article_set.all()
[<Article: NASA uses Python>]
An alternate to calling clear() is to assign the empty set:
>>> p1.article_set = []
>>> p1.article_set.all()
[]
>>> a2.publications = [p1, new_publication]
>>> a2.publications.all()
[<Publication: Highlights for Children>, <Publication: The Python Journal>]
>>> a2.publications = []
>>> a2.publications.all()
[]
Dec 23, 2012