##########################
Many-to-many relationships
##########################

.. highlight:: pycon

To define a many-to-many relationship, use :ref:`ref-manytomany`.

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)

        # On Python 3: def __str__(self):
        def __unicode__(self):
            return self.title

        class Meta:
            ordering = ('title',)

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

        # On Python 3: def __str__(self):
        def __unicode__(self):
            return self.headline

        class Meta:
            ordering = ('headline',)

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

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 :exc:`~exceptions.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()
    [<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>]

Many-to-many relationships can be queried using :ref:`lookups across
relationships <lookups-that-span-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 :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()
    [<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 :class:`~django.db.models.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=[a1,a2]).distinct()
    [<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)
    [<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()
    [<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::

    >>> a4.publications.all()
    [<Publication: Science News>]
    >>> a4.publications = [p3]
    >>> a4.publications.all()
    [<Publication: Science Weekly>]

Relation sets can be cleared::

    >>> p2.article_set.clear()
    >>> p2.article_set.all()
    []

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>]

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()
    [<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 :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)
    []
    >>> p1.article_set.all()
    [<Article: NASA uses Python>]

An alternate to calling
:meth:`~django.db.models.fields.related.RelatedManager.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()
    []