259 lines
9.0 KiB
Python
259 lines
9.0 KiB
Python
"""
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5. Many-to-many relationships
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To define a many-to-many relationship, use ManyToManyField().
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In this example, an article can be published in multiple publications,
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and a publication has multiple articles.
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"""
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from django.db import models
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class Publication(models.Model):
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title = models.CharField(maxlength=30)
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def __str__(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(maxlength=100)
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publications = models.ManyToManyField(Publication)
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def __str__(self):
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return self.headline
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class Meta:
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ordering = ('headline',)
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__test__ = {'API_TESTS':"""
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# Create a couple of Publications.
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>>> p1 = Publication(id=None, title='The Python Journal')
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>>> p1.save()
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>>> p2 = Publication(id=None, title='Science News')
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>>> p2.save()
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>>> p3 = Publication(id=None, title='Science Weekly')
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>>> p3.save()
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# Create an Article.
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>>> a1 = Article(id=None, headline='Django lets you build Web apps easily')
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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(id=None, 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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# 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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# We can perform kwarg queries across m2m 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=[1,p2]).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
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# have 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=[1,a2]).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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# 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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>>> p2.article_set = [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>]
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>>> a4.publications = [p3]
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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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[<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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>>> a4.publications.all()
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[<Publication: Science Weekly>]
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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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# Relation sets can also be set using primary key values
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>>> p2.article_set = [a4.id, a5.id]
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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>]
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>>> a4.publications = [p3.id]
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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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[<Publication: Science Weekly>]
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# Recreate the article and Publication we have deleted.
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>>> p1 = Publication(id=None, title='The Python Journal')
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>>> p1.save()
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>>> a2 = Article(id=None, 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 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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"""}
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