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git-svn-id: http://code.djangoproject.com/svn/django/trunk@3661 bcc190cf-cafb-0310-a4f2-bffc1f526a37
125 lines
3.0 KiB
Python
125 lines
3.0 KiB
Python
"""
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16. Many-to-one relationships that can be null
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To define a many-to-one relationship that can have a null foreign key, use
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``ForeignKey()`` with ``null=True`` .
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"""
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from django.db import models
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class Reporter(models.Model):
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name = models.CharField(maxlength=30)
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def __str__(self):
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return self.name
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class Article(models.Model):
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headline = models.CharField(maxlength=100)
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reporter = models.ForeignKey(Reporter, null=True)
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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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__test__ = {'API_TESTS':"""
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# Create a Reporter.
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>>> r = Reporter(name='John Smith')
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>>> r.save()
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# Create an Article.
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>>> a = Article(headline="First", reporter=r)
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>>> a.save()
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>>> a.reporter.id
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1
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>>> a.reporter
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<Reporter: John Smith>
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# Article objects have access to their related Reporter objects.
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>>> r = a.reporter
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# Create an Article via the Reporter object.
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>>> a2 = r.article_set.create(headline="Second")
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>>> a2
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<Article: Second>
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>>> a2.reporter.id
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1
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# Reporter objects have access to their related Article objects.
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>>> r.article_set.all()
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[<Article: First>, <Article: Second>]
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>>> r.article_set.filter(headline__startswith='Fir')
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[<Article: First>]
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>>> r.article_set.count()
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2
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# Create an Article with no Reporter by passing "reporter=None".
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>>> a3 = Article(headline="Third", reporter=None)
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>>> a3.save()
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>>> a3.id
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3
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>>> print a3.reporter
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None
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# Need to reget a3 to refresh the cache
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>>> a3 = Article.objects.get(pk=3)
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>>> print a3.reporter.id
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Traceback (most recent call last):
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...
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AttributeError: 'NoneType' object has no attribute 'id'
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# Accessing an article's 'reporter' attribute returns None
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# if the reporter is set to None.
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>>> print a3.reporter
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None
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# To retrieve the articles with no reporters set, use "reporter__isnull=True".
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>>> Article.objects.filter(reporter__isnull=True)
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[<Article: Third>]
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# Set the reporter for the Third article
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>>> r.article_set.add(a3)
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>>> r.article_set.all()
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[<Article: First>, <Article: Second>, <Article: Third>]
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# Remove an article from the set, and check that it was removed.
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>>> r.article_set.remove(a3)
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>>> r.article_set.all()
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[<Article: First>, <Article: Second>]
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>>> Article.objects.filter(reporter__isnull=True)
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[<Article: Third>]
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# Create another article and reporter
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>>> r2 = Reporter(name='Paul Jones')
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>>> r2.save()
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>>> a4 = r2.article_set.create(headline='Fourth')
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>>> r2.article_set.all()
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[<Article: Fourth>]
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# Try to remove a4 from a set it does not belong to
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>>> r.article_set.remove(a4)
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Traceback (most recent call last):
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...
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DoesNotExist: <Article: Fourth> is not related to <Reporter: John Smith>.
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>>> r2.article_set.all()
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[<Article: Fourth>]
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# Use descriptor assignment to allocate ForeignKey. Null is legal, so
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# existing members of set that are not in the assignment set are set null
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>>> r2.article_set = [a2, a3]
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>>> r2.article_set.all()
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[<Article: Second>, <Article: Third>]
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# Clear the rest of the set
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>>> r.article_set.clear()
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>>> r.article_set.all()
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[]
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>>> Article.objects.filter(reporter__isnull=True)
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[<Article: First>, <Article: Fourth>]
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"""}
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