{"title":"Many-to-many relationships","version":"4.1","locale":"el","docname":"topics/db/examples/many_to_many","url":"/el/4.1/topics/db/examples/many_to_many/","canonical":"https://djangodocs.dev/el/4.1/topics/db/examples/many_to_many/","summary":"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…","html":"

Many-to-many relationshipsLink to this heading#

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To define a many-to-many relationship, use\nManyToManyField.

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In this example, an Article can be published in multiple Publication\nobjects, and a Publication has multiple Article objects:

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Python
from django.db import models\n\nclass Publication(models.Model):\n    title = models.CharField(max_length=30)\n\n    class Meta:\n        ordering = ['title']\n\n    def __str__(self):\n        return self.title\n\nclass Article(models.Model):\n    headline = models.CharField(max_length=100)\n    publications = models.ManyToManyField(Publication)\n\n    class Meta:\n        ordering = ['headline']\n\n    def __str__(self):\n        return self.headline\n
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What follows are examples of operations that can be performed using the Python\nAPI facilities.

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Create a few Publications:

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Python console
>>> p1 = Publication(title='The Python Journal')\n>>> p1.save()\n>>> p2 = Publication(title='Science News')\n>>> p2.save()\n>>> p3 = Publication(title='Science Weekly')\n>>> p3.save()\n
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Create an Article:

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Python console
>>> a1 = Article(headline='Django lets you build web apps easily')\n
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You can’t associate it with a Publication until it’s been saved:

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Python console
>>> a1.publications.add(p1)\nTraceback (most recent call last):\n...\nValueError: "<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.\n
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Save it!

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Python console
>>> a1.save()\n
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Associate the Article with a Publication:

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Python console
>>> a1.publications.add(p1)\n
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Create another Article, and set it to appear in the Publications:

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Python console
>>> a2 = Article(headline='NASA uses Python')\n>>> a2.save()\n>>> a2.publications.add(p1, p2)\n>>> a2.publications.add(p3)\n
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Adding a second time is OK, it will not duplicate the relation:

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Python console
>>> a2.publications.add(p3)\n
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Adding an object of the wrong type raises TypeError:

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Python console
>>> a2.publications.add(a1)\nTraceback (most recent call last):\n...\nTypeError: 'Publication' instance expected\n
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Create and add a Publication to an Article in one step using\ncreate():

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Python console
>>> new_publication = a2.publications.create(title='Highlights for Children')\n
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Article objects have access to their related Publication objects:

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Python console
>>> a1.publications.all()\n<QuerySet [<Publication: The Python Journal>]>\n>>> a2.publications.all()\n<QuerySet [<Publication: Highlights for Children>, <Publication: Science News>, <Publication: Science Weekly>, <Publication: The Python Journal>]>\n
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Publication objects have access to their related Article objects:

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Python console
>>> p2.article_set.all()\n<QuerySet [<Article: NASA uses Python>]>\n>>> p1.article_set.all()\n<QuerySet [<Article: Django lets you build web apps easily>, <Article: NASA uses Python>]>\n>>> Publication.objects.get(id=4).article_set.all()\n<QuerySet [<Article: NASA uses Python>]>\n
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Many-to-many relationships can be queried using lookups across\nrelationships:

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Python console
>>> Article.objects.filter(publications__id=1)\n<QuerySet [<Article: Django lets you build web apps easily>, <Article: NASA uses Python>]>\n>>> Article.objects.filter(publications__pk=1)\n<QuerySet [<Article: Django lets you build web apps easily>, <Article: NASA uses Python>]>\n>>> Article.objects.filter(publications=1)\n<QuerySet [<Article: Django lets you build web apps easily>, <Article: NASA uses Python>]>\n>>> Article.objects.filter(publications=p1)\n<QuerySet [<Article: Django lets you build web apps easily>, <Article: NASA uses Python>]>\n\n>>> Article.objects.filter(publications__title__startswith="Science")\n<QuerySet [<Article: NASA uses Python>, <Article: NASA uses Python>]>\n\n>>> Article.objects.filter(publications__title__startswith="Science").distinct()\n<QuerySet [<Article: NASA uses Python>]>\n
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The count() function respects\ndistinct() as well:

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Python console
>>> Article.objects.filter(publications__title__startswith="Science").count()\n2\n\n>>> Article.objects.filter(publications__title__startswith="Science").distinct().count()\n1\n\n>>> Article.objects.filter(publications__in=[1,2]).distinct()\n<QuerySet [<Article: Django lets you build web apps easily>, <Article: NASA uses Python>]>\n>>> Article.objects.filter(publications__in=[p1,p2]).distinct()\n<QuerySet [<Article: Django lets you build web apps easily>, <Article: NASA uses Python>]>\n
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Reverse m2m queries are supported (i.e., starting at the table that doesn’t have\na ManyToManyField):

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Python console
>>> Publication.objects.filter(id=1)\n<QuerySet [<Publication: The Python Journal>]>\n>>> Publication.objects.filter(pk=1)\n<QuerySet [<Publication: The Python Journal>]>\n\n>>> Publication.objects.filter(article__headline__startswith="NASA")\n<QuerySet [<Publication: Highlights for Children>, <Publication: Science News>, <Publication: Science Weekly>, <Publication: The Python Journal>]>\n\n>>> Publication.objects.filter(article__id=1)\n<QuerySet [<Publication: The Python Journal>]>\n>>> Publication.objects.filter(article__pk=1)\n<QuerySet [<Publication: The Python Journal>]>\n>>> Publication.objects.filter(article=1)\n<QuerySet [<Publication: The Python Journal>]>\n>>> Publication.objects.filter(article=a1)\n<QuerySet [<Publication: The Python Journal>]>\n\n>>> Publication.objects.filter(article__in=[1,2]).distinct()\n<QuerySet [<Publication: Highlights for Children>, <Publication: Science News>, <Publication: Science Weekly>, <Publication: The Python Journal>]>\n>>> Publication.objects.filter(article__in=[a1,a2]).distinct()\n<QuerySet [<Publication: Highlights for Children>, <Publication: Science News>, <Publication: Science Weekly>, <Publication: The Python Journal>]>\n
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Excluding a related item works as you would expect, too (although the SQL\ninvolved is a little complex):

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Python console
>>> Article.objects.exclude(publications=p2)\n<QuerySet [<Article: Django lets you build web apps easily>]>\n
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If we delete a Publication, its Articles won’t be able to access it:

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Python console
>>> p1.delete()\n>>> Publication.objects.all()\n<QuerySet [<Publication: Highlights for Children>, <Publication: Science News>, <Publication: Science Weekly>]>\n>>> a1 = Article.objects.get(pk=1)\n>>> a1.publications.all()\n<QuerySet []>\n
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If we delete an Article, its Publications won’t be able to access it:

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Python console
>>> a2.delete()\n>>> Article.objects.all()\n<QuerySet [<Article: Django lets you build web apps easily>]>\n>>> p2.article_set.all()\n<QuerySet []>\n
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Adding via the “other” end of an m2m:

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Python console
>>> a4 = Article(headline='NASA finds intelligent life on Earth')\n>>> a4.save()\n>>> p2.article_set.add(a4)\n>>> p2.article_set.all()\n<QuerySet [<Article: NASA finds intelligent life on Earth>]>\n>>> a4.publications.all()\n<QuerySet [<Publication: Science News>]>\n
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Adding via the other end using keywords:

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Python console
>>> new_article = p2.article_set.create(headline='Oxygen-free diet works wonders')\n>>> p2.article_set.all()\n<QuerySet [<Article: NASA finds intelligent life on Earth>, <Article: Oxygen-free diet works wonders>]>\n>>> a5 = p2.article_set.all()[1]\n>>> a5.publications.all()\n<QuerySet [<Publication: Science News>]>\n
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Removing Publication from an Article:

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Python console
>>> a4.publications.remove(p2)\n>>> p2.article_set.all()\n<QuerySet [<Article: Oxygen-free diet works wonders>]>\n>>> a4.publications.all()\n<QuerySet []>\n
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And from the other end:

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Python console
>>> p2.article_set.remove(a5)\n>>> p2.article_set.all()\n<QuerySet []>\n>>> a5.publications.all()\n<QuerySet []>\n
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Relation sets can be set:

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Python console
>>> a4.publications.all()\n<QuerySet [<Publication: Science News>]>\n>>> a4.publications.set([p3])\n>>> a4.publications.all()\n<QuerySet [<Publication: Science Weekly>]>\n
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Relation sets can be cleared:

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Python console
>>> p2.article_set.clear()\n>>> p2.article_set.all()\n<QuerySet []>\n
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And you can clear from the other end:

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Python console
>>> p2.article_set.add(a4, a5)\n>>> p2.article_set.all()\n<QuerySet [<Article: NASA finds intelligent life on Earth>, <Article: Oxygen-free diet works wonders>]>\n>>> a4.publications.all()\n<QuerySet [<Publication: Science News>, <Publication: Science Weekly>]>\n>>> a4.publications.clear()\n>>> a4.publications.all()\n<QuerySet []>\n>>> p2.article_set.all()\n<QuerySet [<Article: Oxygen-free diet works wonders>]>\n
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Recreate the Article and Publication we have deleted:

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Python console
>>> p1 = Publication(title='The Python Journal')\n>>> p1.save()\n>>> a2 = Article(headline='NASA uses Python')\n>>> a2.save()\n>>> a2.publications.add(p1, p2, p3)\n
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Bulk delete some Publications - references to deleted publications should\ngo:

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Python console
>>> Publication.objects.filter(title__startswith='Science').delete()\n>>> Publication.objects.all()\n<QuerySet [<Publication: Highlights for Children>, <Publication: The Python Journal>]>\n>>> Article.objects.all()\n<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>]>\n>>> a2.publications.all()\n<QuerySet [<Publication: The Python Journal>]>\n
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Bulk delete some articles - references to deleted objects should go:

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Python console
>>> q = Article.objects.filter(headline__startswith='Django')\n>>> print(q)\n<QuerySet [<Article: Django lets you build web apps easily>]>\n>>> q.delete()\n
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After the delete(), the\nQuerySet cache needs to be cleared, and the\nreferenced objects should be gone:

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Python console
>>> print(q)\n<QuerySet []>\n>>> p1.article_set.all()\n<QuerySet [<Article: NASA uses Python>]>\n
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