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290 lines
9.5 KiB
Plaintext
290 lines
9.5 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 :ref:`ref-manytomany`.
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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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def __unicode__(self):
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return self.title
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class Meta:
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ordering = ('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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def __unicode__(self):
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return self.headline
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class Meta:
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ordering = ('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 couple of 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' instance needs to have a primary key value before a 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 both 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::
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>>> a2.publications.add(p3)
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Adding an object of the wrong type raises 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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Add a Publication directly via publications.add by using keyword arguments::
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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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[<Publication: The Python Journal>]
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>>> a2.publications.all()
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[<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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[<Article: NASA uses Python>]
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>>> p1.article_set.all()
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[<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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[<Article: NASA uses Python>]
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Many-to-many relationships can be queried using :ref:`lookups across relationships <lookups-that-span-relationships>`::
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>>> Article.objects.filter(publications__id__exact=1)
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[<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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[<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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[<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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[<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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[<Article: NASA uses Python>, <Article: NASA uses Python>]
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>>> Article.objects.filter(publications__title__startswith="Science").distinct()
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[<Article: NASA uses Python>]
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The count() function respects 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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[<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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[<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 ManyToManyField)::
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>>> Publication.objects.filter(id__exact=1)
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[<Publication: The Python Journal>]
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>>> Publication.objects.filter(pk=1)
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[<Publication: The Python Journal>]
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>>> Publication.objects.filter(article__headline__startswith="NASA")
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[<Publication: Highlights for Children>, <Publication: Science News>, <Publication: Science Weekly>, <Publication: The Python Journal>]
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>>> Publication.objects.filter(article__id__exact=1)
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[<Publication: The Python Journal>]
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>>> Publication.objects.filter(article__pk=1)
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[<Publication: The Python Journal>]
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>>> Publication.objects.filter(article=1)
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[<Publication: The Python Journal>]
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>>> Publication.objects.filter(article=a1)
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[<Publication: The Python Journal>]
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>>> Publication.objects.filter(article__in=[1,2]).distinct()
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[<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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[<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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[<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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[<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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[]
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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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[<Article: Django lets you build Web apps easily>]
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>>> p2.article_set.all()
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[]
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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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[<Article: NASA finds intelligent life on Earth>]
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>>> a4.publications.all()
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[<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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[<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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[<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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[<Article: Oxygen-free diet works wonders>]
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>>> a4.publications.all()
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[]
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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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[]
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>>> a5.publications.all()
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[]
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Relation sets can be assigned. Assignment clears any existing set members::
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>>> a4.publications.all()
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[<Publication: Science News>]
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>>> a4.publications = [p3]
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>>> a4.publications.all()
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[<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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[]
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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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[<Article: NASA finds intelligent life on Earth>, <Article: Oxygen-free diet works wonders>]
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>>> a4.publications.all()
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[<Publication: Science News>, <Publication: Science Weekly>]
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>>> a4.publications.clear()
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>>> a4.publications.all()
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[]
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>>> p2.article_set.all()
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[<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 go::
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>>> Publication.objects.filter(title__startswith='Science').delete()
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>>> Publication.objects.all()
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[<Publication: Highlights for Children>, <Publication: The Python Journal>]
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>>> Article.objects.all()
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[<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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[<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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[<Article: Django lets you build Web apps easily>]
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>>> q.delete()
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After the delete, the QuerySet cache needs to be cleared, and the referenced
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objects should be gone::
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>>> print(q)
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[]
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>>> p1.article_set.all()
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[<Article: NASA uses Python>]
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An alternate to calling clear() is to assign the empty set::
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>>> p1.article_set = []
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>>> p1.article_set.all()
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[]
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>>> a2.publications = [p1, new_publication]
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>>> a2.publications.all()
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[<Publication: Highlights for Children>, <Publication: The Python Journal>]
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>>> a2.publications = []
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>>> a2.publications.all()
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[]
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