5. Many-to-many relationships

This example describes Django version 0.96. For the current example, go here.

To define a many-to-many relationship, use ManyToManyField().

In this example, an article can be published in multiple publications, and a publication has multiple articles.

Model source code

from django.db import models

class Publication(models.Model):
    title = models.CharField(maxlength=30)

    def __str__(self):
        return self.title

    class Meta:
        ordering = ('title',)

class Article(models.Model):
    headline = models.CharField(maxlength=100)
    publications = models.ManyToManyField(Publication)

    def __str__(self):
        return self.headline

    class Meta:
        ordering = ('headline',)

Sample API usage

This sample code assumes the above models have been saved in a file mysite/models.py.

>>> from mysite.models import Publication, Article

# Create a couple of Publications.
>>> p1 = Publication(id=None, title='The Python Journal')
>>> p1.save()
>>> p2 = Publication(id=None, title='Science News')
>>> p2.save()
>>> p3 = Publication(id=None, title='Science Weekly')
>>> p3.save()

# Create an Article.
>>> a1 = Article(id=None, headline='Django lets you build Web apps easily')
>>> 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(id=None, 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)

# 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>]

# We can perform kwarg queries across m2m 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=[1,p2]).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=[1,a2]).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>]

# 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
>>> p2.article_set = [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>]
>>> a4.publications = [p3]
>>> p2.article_set.all()
[<Article: Oxygen-free diet works wonders>]
>>> a4.publications.all()
[<Publication: Science Weekly>]

# Relation sets can be cleared:
>>> p2.article_set.clear()
>>> p2.article_set.all()
[]
>>> a4.publications.all()
[<Publication: Science Weekly>]

# 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>]

# Relation sets can also be set using primary key values
>>> p2.article_set = [a4.id, a5.id]
>>> p2.article_set.all()
[<Article: NASA finds intelligent life on Earth>, <Article: Oxygen-free diet works wonders>]
>>> a4.publications.all()
[<Publication: Science News>]
>>> a4.publications = [p3.id]
>>> p2.article_set.all()
[<Article: Oxygen-free diet works wonders>]
>>> a4.publications.all()
[<Publication: Science Weekly>]

# Recreate the article and Publication we have deleted.
>>> p1 = Publication(id=None, title='The Python Journal')
>>> p1.save()
>>> a2 = Article(id=None, 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()
[]