Many-to-many Relationships - Django Documentation

Many-to-many relationships¶

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) class Meta: ordering = ['title'] def __str__(self): return self.title class Article(models.Model): headline = models.CharField(max_length=100) publications = models.ManyToManyField(Publication) class Meta: ordering = ['headline'] def __str__(self): return self.headline

What follows are examples of operations that can be performed using the Python API facilities.

Create a few 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: Django lets you build web apps easily>" needs to have a value for field "id" before this 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 the Publications:

>>> a2 = Article(headline='NASA uses Python') >>> a2.save() >>> a2.publications.add(p1, p2) >>> a2.publications.add(p3)

Adding a second time is OK, it will not duplicate the relation:

>>> 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>]>