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209 lines
7.4 KiB
Plaintext
209 lines
7.4 KiB
Plaintext
=========================
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Many-to-one relationships
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=========================
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To define a many-to-one relationship, use :class:`~django.db.models.ForeignKey`::
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from django.db import models
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class Reporter(models.Model):
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first_name = models.CharField(max_length=30)
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last_name = models.CharField(max_length=30)
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email = models.EmailField()
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def __str__(self):
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return "%s %s" % (self.first_name, self.last_name)
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class Article(models.Model):
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headline = models.CharField(max_length=100)
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pub_date = models.DateField()
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reporter = models.ForeignKey(Reporter, on_delete=models.CASCADE)
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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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What follows are examples of operations that can be performed using the Python
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API facilities.
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.. highlight:: pycon
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Create a few Reporters::
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>>> r = Reporter(first_name='John', last_name='Smith', email='john@example.com')
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>>> r.save()
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>>> r2 = Reporter(first_name='Paul', last_name='Jones', email='paul@example.com')
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>>> r2.save()
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Create an Article::
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>>> from datetime import date
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>>> a = Article(id=None, headline="This is a test", pub_date=date(2005, 7, 27), 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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Note that you must save an object before it can be assigned to a foreign key
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relationship. For example, creating an ``Article`` with unsaved ``Reporter``
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raises ``ValueError``::
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>>> r3 = Reporter(first_name='John', last_name='Smith', email='john@example.com')
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>>> Article.objects.create(headline="This is a test", pub_date=date(2005, 7, 27), reporter=r3)
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Traceback (most recent call last):
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...
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ValueError: save() prohibited to prevent data loss due to unsaved related object 'reporter'.
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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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>>> new_article = r.article_set.create(headline="John's second story", pub_date=date(2005, 7, 29))
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>>> new_article
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<Article: John's second story>
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>>> new_article.reporter
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<Reporter: John Smith>
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>>> new_article.reporter.id
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1
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Create a new article::
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>>> new_article2 = Article.objects.create(headline="Paul's story", pub_date=date(2006, 1, 17), reporter=r)
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>>> new_article2.reporter
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<Reporter: John Smith>
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>>> new_article2.reporter.id
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1
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>>> r.article_set.all()
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<QuerySet [<Article: John's second story>, <Article: Paul's story>, <Article: This is a test>]>
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Add the same article to a different article set - check that it moves::
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>>> r2.article_set.add(new_article2)
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>>> new_article2.reporter.id
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2
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>>> new_article2.reporter
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<Reporter: Paul Jones>
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Adding an object of the wrong type raises TypeError::
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>>> r.article_set.add(r2)
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Traceback (most recent call last):
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...
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TypeError: 'Article' instance expected, got <Reporter: Paul Jones>
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>>> r.article_set.all()
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<QuerySet [<Article: John's second story>, <Article: This is a test>]>
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>>> r2.article_set.all()
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<QuerySet [<Article: Paul's story>]>
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>>> r.article_set.count()
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2
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>>> r2.article_set.count()
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1
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Note that in the last example the article has moved from John to Paul.
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Related managers support field lookups as well.
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The API automatically follows relationships as far as you need.
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Use double underscores to separate relationships.
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This works as many levels deep as you want. There's no limit. For example::
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>>> r.article_set.filter(headline__startswith='This')
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<QuerySet [<Article: This is a test>]>
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# Find all Articles for any Reporter whose first name is "John".
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>>> Article.objects.filter(reporter__first_name='John')
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<QuerySet [<Article: John's second story>, <Article: This is a test>]>
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Exact match is implied here::
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>>> Article.objects.filter(reporter__first_name='John')
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<QuerySet [<Article: John's second story>, <Article: This is a test>]>
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Query twice over the related field. This translates to an AND condition in the
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WHERE clause::
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>>> Article.objects.filter(reporter__first_name='John', reporter__last_name='Smith')
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<QuerySet [<Article: John's second story>, <Article: This is a test>]>
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For the related lookup you can supply a primary key value or pass the related
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object explicitly::
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>>> Article.objects.filter(reporter__pk=1)
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<QuerySet [<Article: John's second story>, <Article: This is a test>]>
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>>> Article.objects.filter(reporter=1)
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<QuerySet [<Article: John's second story>, <Article: This is a test>]>
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>>> Article.objects.filter(reporter=r)
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<QuerySet [<Article: John's second story>, <Article: This is a test>]>
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>>> Article.objects.filter(reporter__in=[1,2]).distinct()
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<QuerySet [<Article: John's second story>, <Article: Paul's story>, <Article: This is a test>]>
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>>> Article.objects.filter(reporter__in=[r,r2]).distinct()
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<QuerySet [<Article: John's second story>, <Article: Paul's story>, <Article: This is a test>]>
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You can also use a queryset instead of a literal list of instances::
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>>> Article.objects.filter(reporter__in=Reporter.objects.filter(first_name='John')).distinct()
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<QuerySet [<Article: John's second story>, <Article: This is a test>]>
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Querying in the opposite direction::
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>>> Reporter.objects.filter(article__pk=1)
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<QuerySet [<Reporter: John Smith>]>
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>>> Reporter.objects.filter(article=1)
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<QuerySet [<Reporter: John Smith>]>
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>>> Reporter.objects.filter(article=a)
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<QuerySet [<Reporter: John Smith>]>
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>>> Reporter.objects.filter(article__headline__startswith='This')
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<QuerySet [<Reporter: John Smith>, <Reporter: John Smith>, <Reporter: John Smith>]>
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>>> Reporter.objects.filter(article__headline__startswith='This').distinct()
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<QuerySet [<Reporter: John Smith>]>
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Counting in the opposite direction works in conjunction with ``distinct()``::
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>>> Reporter.objects.filter(article__headline__startswith='This').count()
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3
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>>> Reporter.objects.filter(article__headline__startswith='This').distinct().count()
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1
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Queries can go round in circles::
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>>> Reporter.objects.filter(article__reporter__first_name__startswith='John')
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<QuerySet [<Reporter: John Smith>, <Reporter: John Smith>, <Reporter: John Smith>, <Reporter: John Smith>]>
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>>> Reporter.objects.filter(article__reporter__first_name__startswith='John').distinct()
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<QuerySet [<Reporter: John Smith>]>
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>>> Reporter.objects.filter(article__reporter=r).distinct()
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<QuerySet [<Reporter: John Smith>]>
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If you delete a reporter, their articles will be deleted (assuming that the
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ForeignKey was defined with :attr:`django.db.models.ForeignKey.on_delete` set to
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``CASCADE``, which is the default)::
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>>> Article.objects.all()
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<QuerySet [<Article: John's second story>, <Article: Paul's story>, <Article: This is a test>]>
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>>> Reporter.objects.order_by('first_name')
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<QuerySet [<Reporter: John Smith>, <Reporter: Paul Jones>]>
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>>> r2.delete()
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>>> Article.objects.all()
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<QuerySet [<Article: John's second story>, <Article: This is a test>]>
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>>> Reporter.objects.order_by('first_name')
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<QuerySet [<Reporter: John Smith>]>
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You can delete using a JOIN in the query::
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>>> Reporter.objects.filter(article__headline__startswith='This').delete()
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>>> Reporter.objects.all()
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<QuerySet []>
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>>> Article.objects.all()
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<QuerySet []>
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