mirror of
https://github.com/django/django.git
synced 2024-12-27 03:25:58 +00:00
338 lines
10 KiB
Plaintext
338 lines
10 KiB
Plaintext
==========================
|
|
Many-to-many relationships
|
|
==========================
|
|
|
|
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``:
|
|
|
|
.. code-block:: pycon
|
|
|
|
>>> 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``:
|
|
|
|
.. code-block:: pycon
|
|
|
|
>>> a1 = Article(headline="Django lets you build web apps easily")
|
|
|
|
You can't associate it with a ``Publication`` until it's been saved:
|
|
|
|
.. code-block:: pycon
|
|
|
|
>>> 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!
|
|
|
|
.. code-block:: pycon
|
|
|
|
>>> a1.save()
|
|
|
|
Associate the ``Article`` with a ``Publication``:
|
|
|
|
.. code-block:: pycon
|
|
|
|
>>> a1.publications.add(p1)
|
|
|
|
Create another ``Article``, and set it to appear in the ``Publications``:
|
|
|
|
.. code-block:: pycon
|
|
|
|
>>> 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:
|
|
|
|
.. code-block:: pycon
|
|
|
|
>>> a2.publications.add(p3)
|
|
|
|
Adding an object of the wrong type raises :exc:`TypeError`:
|
|
|
|
.. code-block:: pycon
|
|
|
|
>>> 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`:
|
|
|
|
.. code-block:: pycon
|
|
|
|
>>> new_publication = a2.publications.create(title="Highlights for Children")
|
|
|
|
``Article`` objects have access to their related ``Publication`` objects:
|
|
|
|
.. code-block:: pycon
|
|
|
|
>>> 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:
|
|
|
|
.. code-block:: pycon
|
|
|
|
>>> 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>`:
|
|
|
|
.. code-block:: pycon
|
|
|
|
>>> 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:
|
|
|
|
.. code-block:: pycon
|
|
|
|
>>> 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`):
|
|
|
|
.. code-block:: pycon
|
|
|
|
>>> 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):
|
|
|
|
.. code-block:: pycon
|
|
|
|
>>> 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:
|
|
|
|
.. code-block:: pycon
|
|
|
|
>>> 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:
|
|
|
|
.. code-block:: pycon
|
|
|
|
>>> 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:
|
|
|
|
.. code-block:: pycon
|
|
|
|
>>> 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:
|
|
|
|
.. code-block:: pycon
|
|
|
|
>>> 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``:
|
|
|
|
.. code-block:: pycon
|
|
|
|
>>> a4.publications.remove(p2)
|
|
>>> p2.article_set.all()
|
|
<QuerySet [<Article: Oxygen-free diet works wonders>]>
|
|
>>> a4.publications.all()
|
|
<QuerySet []>
|
|
|
|
And from the other end:
|
|
|
|
.. code-block:: pycon
|
|
|
|
>>> p2.article_set.remove(a5)
|
|
>>> p2.article_set.all()
|
|
<QuerySet []>
|
|
>>> a5.publications.all()
|
|
<QuerySet []>
|
|
|
|
Relation sets can be set:
|
|
|
|
.. code-block:: pycon
|
|
|
|
>>> a4.publications.all()
|
|
<QuerySet [<Publication: Science News>]>
|
|
>>> a4.publications.set([p3])
|
|
>>> a4.publications.all()
|
|
<QuerySet [<Publication: Science Weekly>]>
|
|
|
|
Relation sets can be cleared:
|
|
|
|
.. code-block:: pycon
|
|
|
|
>>> p2.article_set.clear()
|
|
>>> p2.article_set.all()
|
|
<QuerySet []>
|
|
|
|
And you can clear from the other end:
|
|
|
|
.. code-block:: pycon
|
|
|
|
>>> 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:
|
|
|
|
.. code-block:: pycon
|
|
|
|
>>> 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:
|
|
|
|
.. code-block:: pycon
|
|
|
|
>>> 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:
|
|
|
|
.. code-block:: pycon
|
|
|
|
>>> 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:
|
|
|
|
.. code-block:: pycon
|
|
|
|
>>> print(q)
|
|
<QuerySet []>
|
|
>>> p1.article_set.all()
|
|
<QuerySet [<Article: NASA uses Python>]>
|