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209 lines
6.9 KiB
Plaintext
209 lines
6.9 KiB
Plaintext
#########################
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Many-to-one relationships
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#########################
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.. highlight:: pycon
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To define a many-to-one relationship, use :class:`~django.db.models.ForeignKey`.
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.. code-block:: python
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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): # __unicode__ on Python 2
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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)
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def __str__(self): # __unicode__ on Python 2
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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 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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Article objects have access to their related Reporter objects::
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>>> r = a.reporter
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On Python 2, these are strings of type ``str`` instead of unicode strings
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because that's what was used in the creation of this reporter (and we haven't
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refreshed the data from the database, which always returns unicode strings)::
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>>> r.first_name, r.last_name
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('John', 'Smith')
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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, and add it to the article set::
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>>> new_article2 = Article(headline="Paul's story", pub_date=date(2006, 1, 17))
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>>> r.article_set.add(new_article2)
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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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[<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
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>>> r.article_set.all()
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[<Article: John's second story>, <Article: This is a test>]
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>>> r2.article_set.all()
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[<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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[<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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[<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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[<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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[<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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[<Article: John's second story>, <Article: This is a test>]
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>>> Article.objects.filter(reporter=1)
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[<Article: John's second story>, <Article: This is a test>]
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>>> Article.objects.filter(reporter=r)
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[<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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[<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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[<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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[<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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[<Reporter: John Smith>]
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>>> Reporter.objects.filter(article=1)
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[<Reporter: John Smith>]
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>>> Reporter.objects.filter(article=a)
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[<Reporter: John Smith>]
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>>> Reporter.objects.filter(article__headline__startswith='This')
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[<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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[<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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[<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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[<Reporter: John Smith>]
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>>> Reporter.objects.filter(article__reporter=r).distinct()
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[<Reporter: John Smith>]
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If you delete a reporter, his 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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[<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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[<Reporter: John Smith>, <Reporter: Paul Jones>]
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>>> r2.delete()
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>>> Article.objects.all()
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[<Article: John's second story>, <Article: This is a test>]
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>>> Reporter.objects.order_by('first_name')
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[<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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[]
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>>> Article.objects.all()
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[]
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