==========================

Many-to-many relationships

==========================

.. highlight:: pycon

To define a many-to-many relationship, use

:class:`~django.db.models.ManyToManyField`.

In this example, an ``Article`` can be published in multiple ``Publication``

objects, and a ``Publication`` has multiple ``Article`` objects:

.. code-block:: python

    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 :exc:`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

:meth:`~django.db.models.fields.related.RelatedManager.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 :ref:`lookups across

relationships <lookups-that-span-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 :meth:`~django.db.models.query.QuerySet.count` function respects

:meth:`~django.db.models.query.QuerySet.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 :class:`~django.db.models.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 :meth:`~django.db.models.query.QuerySet.delete`, the

:class:`~django.db.models.query.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>]>