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283 lines
10 KiB
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283 lines
10 KiB
Plaintext
==========================
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Many-to-many relationships
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==========================
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.. highlight:: pycon
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To define a many-to-many relationship, use
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:class:`~django.db.models.ManyToManyField`.
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In this example, an ``Article`` can be published in multiple ``Publication``
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objects, and a ``Publication`` has multiple ``Article`` objects:
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.. code-block:: python
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from django.db import models
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class Publication(models.Model):
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title = models.CharField(max_length=30)
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class Meta:
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ordering = ['title']
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def __str__(self):
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return self.title
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class Article(models.Model):
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headline = models.CharField(max_length=100)
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publications = models.ManyToManyField(Publication)
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class Meta:
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ordering = ['headline']
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def __str__(self):
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return self.headline
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What follows are examples of operations that can be performed using the Python
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API facilities.
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Create a few ``Publications``::
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>>> p1 = Publication(title='The Python Journal')
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>>> p1.save()
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>>> p2 = Publication(title='Science News')
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>>> p2.save()
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>>> p3 = Publication(title='Science Weekly')
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>>> p3.save()
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Create an ``Article``::
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>>> a1 = Article(headline='Django lets you build Web apps easily')
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You can't associate it with a ``Publication`` until it's been saved::
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>>> a1.publications.add(p1)
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Traceback (most recent call last):
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...
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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.
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Save it!
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::
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>>> a1.save()
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Associate the ``Article`` with a ``Publication``::
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>>> a1.publications.add(p1)
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Create another ``Article``, and set it to appear in the ``Publications``::
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>>> a2 = Article(headline='NASA uses Python')
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>>> a2.save()
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>>> a2.publications.add(p1, p2)
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>>> a2.publications.add(p3)
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Adding a second time is OK, it will not duplicate the relation::
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>>> a2.publications.add(p3)
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Adding an object of the wrong type raises :exc:`TypeError`::
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>>> a2.publications.add(a1)
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Traceback (most recent call last):
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...
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TypeError: 'Publication' instance expected
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Create and add a ``Publication`` to an ``Article`` in one step using
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:meth:`~django.db.models.fields.related.RelatedManager.create`::
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>>> new_publication = a2.publications.create(title='Highlights for Children')
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``Article`` objects have access to their related ``Publication`` objects::
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>>> a1.publications.all()
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<QuerySet [<Publication: The Python Journal>]>
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>>> a2.publications.all()
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<QuerySet [<Publication: Highlights for Children>, <Publication: Science News>, <Publication: Science Weekly>, <Publication: The Python Journal>]>
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``Publication`` objects have access to their related ``Article`` objects::
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>>> p2.article_set.all()
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<QuerySet [<Article: NASA uses Python>]>
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>>> p1.article_set.all()
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<QuerySet [<Article: Django lets you build Web apps easily>, <Article: NASA uses Python>]>
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>>> Publication.objects.get(id=4).article_set.all()
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<QuerySet [<Article: NASA uses Python>]>
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Many-to-many relationships can be queried using :ref:`lookups across
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relationships <lookups-that-span-relationships>`::
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>>> Article.objects.filter(publications__id=1)
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<QuerySet [<Article: Django lets you build Web apps easily>, <Article: NASA uses Python>]>
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>>> Article.objects.filter(publications__pk=1)
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<QuerySet [<Article: Django lets you build Web apps easily>, <Article: NASA uses Python>]>
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>>> Article.objects.filter(publications=1)
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<QuerySet [<Article: Django lets you build Web apps easily>, <Article: NASA uses Python>]>
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>>> Article.objects.filter(publications=p1)
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<QuerySet [<Article: Django lets you build Web apps easily>, <Article: NASA uses Python>]>
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>>> Article.objects.filter(publications__title__startswith="Science")
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<QuerySet [<Article: NASA uses Python>, <Article: NASA uses Python>]>
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>>> Article.objects.filter(publications__title__startswith="Science").distinct()
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<QuerySet [<Article: NASA uses Python>]>
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The :meth:`~django.db.models.query.QuerySet.count` function respects
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:meth:`~django.db.models.query.QuerySet.distinct` as well::
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>>> Article.objects.filter(publications__title__startswith="Science").count()
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2
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>>> Article.objects.filter(publications__title__startswith="Science").distinct().count()
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1
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>>> Article.objects.filter(publications__in=[1,2]).distinct()
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<QuerySet [<Article: Django lets you build Web apps easily>, <Article: NASA uses Python>]>
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>>> Article.objects.filter(publications__in=[p1,p2]).distinct()
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<QuerySet [<Article: Django lets you build Web apps easily>, <Article: NASA uses Python>]>
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Reverse m2m queries are supported (i.e., starting at the table that doesn't have
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a :class:`~django.db.models.ManyToManyField`)::
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>>> Publication.objects.filter(id=1)
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<QuerySet [<Publication: The Python Journal>]>
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>>> Publication.objects.filter(pk=1)
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<QuerySet [<Publication: The Python Journal>]>
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>>> Publication.objects.filter(article__headline__startswith="NASA")
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<QuerySet [<Publication: Highlights for Children>, <Publication: Science News>, <Publication: Science Weekly>, <Publication: The Python Journal>]>
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>>> Publication.objects.filter(article__id=1)
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<QuerySet [<Publication: The Python Journal>]>
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>>> Publication.objects.filter(article__pk=1)
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<QuerySet [<Publication: The Python Journal>]>
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>>> Publication.objects.filter(article=1)
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<QuerySet [<Publication: The Python Journal>]>
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>>> Publication.objects.filter(article=a1)
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<QuerySet [<Publication: The Python Journal>]>
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>>> Publication.objects.filter(article__in=[1,2]).distinct()
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<QuerySet [<Publication: Highlights for Children>, <Publication: Science News>, <Publication: Science Weekly>, <Publication: The Python Journal>]>
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>>> Publication.objects.filter(article__in=[a1,a2]).distinct()
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<QuerySet [<Publication: Highlights for Children>, <Publication: Science News>, <Publication: Science Weekly>, <Publication: The Python Journal>]>
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Excluding a related item works as you would expect, too (although the SQL
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involved is a little complex)::
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>>> Article.objects.exclude(publications=p2)
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<QuerySet [<Article: Django lets you build Web apps easily>]>
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If we delete a ``Publication``, its ``Articles`` won't be able to access it::
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>>> p1.delete()
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>>> Publication.objects.all()
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<QuerySet [<Publication: Highlights for Children>, <Publication: Science News>, <Publication: Science Weekly>]>
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>>> a1 = Article.objects.get(pk=1)
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>>> a1.publications.all()
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<QuerySet []>
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If we delete an ``Article``, its ``Publications`` won't be able to access it::
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>>> a2.delete()
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>>> Article.objects.all()
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<QuerySet [<Article: Django lets you build Web apps easily>]>
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>>> p2.article_set.all()
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<QuerySet []>
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Adding via the 'other' end of an m2m::
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>>> a4 = Article(headline='NASA finds intelligent life on Earth')
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>>> a4.save()
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>>> p2.article_set.add(a4)
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>>> p2.article_set.all()
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<QuerySet [<Article: NASA finds intelligent life on Earth>]>
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>>> a4.publications.all()
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<QuerySet [<Publication: Science News>]>
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Adding via the other end using keywords::
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>>> new_article = p2.article_set.create(headline='Oxygen-free diet works wonders')
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>>> p2.article_set.all()
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<QuerySet [<Article: NASA finds intelligent life on Earth>, <Article: Oxygen-free diet works wonders>]>
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>>> a5 = p2.article_set.all()[1]
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>>> a5.publications.all()
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<QuerySet [<Publication: Science News>]>
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Removing ``Publication`` from an ``Article``::
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>>> a4.publications.remove(p2)
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>>> p2.article_set.all()
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<QuerySet [<Article: Oxygen-free diet works wonders>]>
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>>> a4.publications.all()
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<QuerySet []>
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And from the other end::
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>>> p2.article_set.remove(a5)
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>>> p2.article_set.all()
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<QuerySet []>
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>>> a5.publications.all()
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<QuerySet []>
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Relation sets can be set::
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>>> a4.publications.all()
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<QuerySet [<Publication: Science News>]>
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>>> a4.publications.set([p3])
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>>> a4.publications.all()
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<QuerySet [<Publication: Science Weekly>]>
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Relation sets can be cleared::
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>>> p2.article_set.clear()
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>>> p2.article_set.all()
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<QuerySet []>
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And you can clear from the other end::
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>>> p2.article_set.add(a4, a5)
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>>> p2.article_set.all()
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<QuerySet [<Article: NASA finds intelligent life on Earth>, <Article: Oxygen-free diet works wonders>]>
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>>> a4.publications.all()
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<QuerySet [<Publication: Science News>, <Publication: Science Weekly>]>
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>>> a4.publications.clear()
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>>> a4.publications.all()
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<QuerySet []>
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>>> p2.article_set.all()
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<QuerySet [<Article: Oxygen-free diet works wonders>]>
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Recreate the ``Article`` and ``Publication`` we have deleted::
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>>> p1 = Publication(title='The Python Journal')
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>>> p1.save()
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>>> a2 = Article(headline='NASA uses Python')
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>>> a2.save()
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>>> a2.publications.add(p1, p2, p3)
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Bulk delete some ``Publications`` - references to deleted publications should
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go::
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>>> Publication.objects.filter(title__startswith='Science').delete()
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>>> Publication.objects.all()
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<QuerySet [<Publication: Highlights for Children>, <Publication: The Python Journal>]>
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>>> Article.objects.all()
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<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>]>
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>>> a2.publications.all()
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<QuerySet [<Publication: The Python Journal>]>
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Bulk delete some articles - references to deleted objects should go::
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>>> q = Article.objects.filter(headline__startswith='Django')
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>>> print(q)
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<QuerySet [<Article: Django lets you build Web apps easily>]>
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>>> q.delete()
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After the :meth:`~django.db.models.query.QuerySet.delete`, the
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:class:`~django.db.models.query.QuerySet` cache needs to be cleared, and the
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referenced objects should be gone::
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>>> print(q)
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<QuerySet []>
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>>> p1.article_set.all()
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<QuerySet [<Article: NASA uses Python>]>
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