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 __str__(self): # __unicode__ on Python 2 return self.title class Meta: ordering = ('title',) class Article(models.Model): headline = models.CharField(max_length=100) publications = models.ManyToManyField(Publication) def __str__(self): # __unicode__ on Python 2 return self.headline class Meta: ordering = ('headline',)
What follows are examples of operations that can be performed using the Python API facilities. Note that if you are using an intermediate model for a many-to-many relationship, some of the related manager’s methods are disabled, so some of these examples won’t work with such models.
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
Create and add a Publication
to an Article
in one step using create()
:
>>> new_publication = a2.publications.create(title='Highlights for Children')
Article
objects have access to their related Publication
objects:
>>> a1.publications.all() <QuerySet [<Publication: The Python Journal>]> >>> a2.publications.all() <QuerySet [<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() <QuerySet [<Article: NASA uses Python>]> >>> p1.article_set.all() <QuerySet [<Article: Django lets you build Web apps easily>, <Article: NASA uses Python>]> >>> Publication.objects.get(id=4).article_set.all() <QuerySet [<Article: NASA uses Python>]>
Many-to-many relationships can be queried using lookups across relationships:
>>> Article.objects.filter(publications__id=1) <QuerySet [<Article: Django lets you build Web apps easily>, <Article: NASA uses Python>]> >>> Article.objects.filter(publications__pk=1) <QuerySet [<Article: Django lets you build Web apps easily>, <Article: NASA uses Python>]> >>> Article.objects.filter(publications=1) <QuerySet [<Article: Django lets you build Web apps easily>, <Article: NASA uses Python>]> >>> Article.objects.filter(publications=p1) <QuerySet [<Article: Django lets you build Web apps easily>, <Article: NASA uses Python>]> >>> Article.objects.filter(publications__title__startswith="Science") <QuerySet [<Article: NASA uses Python>, <Article: NASA uses Python>]> >>> Article.objects.filter(publications__title__startswith="Science").distinct() <QuerySet [<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() <QuerySet [<Article: Django lets you build Web apps easily>, <Article: NASA uses Python>]> >>> Article.objects.filter(publications__in=[p1,p2]).distinct() <QuerySet [<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=1) <QuerySet [<Publication: The Python Journal>]> >>> Publication.objects.filter(pk=1) <QuerySet [<Publication: The Python Journal>]> >>> Publication.objects.filter(article__headline__startswith="NASA") <QuerySet [<Publication: Highlights for Children>, <Publication: Science News>, <Publication: Science Weekly>, <Publication: The Python Journal>]> >>> Publication.objects.filter(article__id=1) <QuerySet [<Publication: The Python Journal>]> >>> Publication.objects.filter(article__pk=1) <QuerySet [<Publication: The Python Journal>]> >>> Publication.objects.filter(article=1) <QuerySet [<Publication: The Python Journal>]> >>> Publication.objects.filter(article=a1) <QuerySet [<Publication: The Python Journal>]> >>> Publication.objects.filter(article__in=[1,2]).distinct() <QuerySet [<Publication: Highlights for Children>, <Publication: Science News>, <Publication: Science Weekly>, <Publication: The Python Journal>]> >>> Publication.objects.filter(article__in=[a1,a2]).distinct() <QuerySet [<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) <QuerySet [<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() <QuerySet [<Publication: Highlights for Children>, <Publication: Science News>, <Publication: Science Weekly>]> >>> a1 = Article.objects.get(pk=1) >>> a1.publications.all() <QuerySet []>
If we delete an Article
, its Publications
won’t be able to access it:
>>> a2.delete() >>> Article.objects.all() <QuerySet [<Article: Django lets you build Web apps easily>]> >>> p2.article_set.all() <QuerySet []>
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() <QuerySet [<Article: NASA finds intelligent life on Earth>]> >>> a4.publications.all() <QuerySet [<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() <QuerySet [<Article: NASA finds intelligent life on Earth>, <Article: Oxygen-free diet works wonders>]> >>> a5 = p2.article_set.all()[1] >>> a5.publications.all() <QuerySet [<Publication: Science News>]>
Removing Publication
from an Article
:
>>> a4.publications.remove(p2) >>> p2.article_set.all() <QuerySet [<Article: Oxygen-free diet works wonders>]> >>> a4.publications.all() <QuerySet []>
And from the other end:
>>> p2.article_set.remove(a5) >>> p2.article_set.all() <QuerySet []> >>> a5.publications.all() <QuerySet []>
Relation sets can be set:
>>> a4.publications.all() <QuerySet [<Publication: Science News>]> >>> a4.publications.set([p3]) >>> a4.publications.all() <QuerySet [<Publication: Science Weekly>]>
Relation sets can be cleared:
>>> p2.article_set.clear() >>> p2.article_set.all() <QuerySet []>
And you can clear from the other end:
>>> p2.article_set.add(a4, a5) >>> p2.article_set.all() <QuerySet [<Article: NASA finds intelligent life on Earth>, <Article: Oxygen-free diet works wonders>]> >>> a4.publications.all() <QuerySet [<Publication: Science News>, <Publication: Science Weekly>]> >>> a4.publications.clear() >>> a4.publications.all() <QuerySet []> >>> p2.article_set.all() <QuerySet [<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() <QuerySet [<Publication: Highlights for Children>, <Publication: The Python Journal>]> >>> Article.objects.all() <QuerySet [<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() <QuerySet [<Publication: The Python Journal>]>
Bulk delete some articles - references to deleted objects should go:
>>> q = Article.objects.filter(headline__startswith='Django') >>> print(q) <QuerySet [<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) <QuerySet []> >>> p1.article_set.all() <QuerySet [<Article: NASA uses Python>]>
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