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112 lines
3.3 KiB
Python
112 lines
3.3 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 __repr__(self):
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return self.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 __repr__(self):
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return self.headline
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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 one Publication. set_publications() returns a
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# boolean, representing whether any records were added or deleted.
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>>> a1.set_publications([p1.id])
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True
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# If we set it again, it'll return False, because the list of Publications
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# hasn't changed.
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>>> a1.set_publications([p1.id])
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False
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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.set_publications([p1.id, p2.id])
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True
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>>> a2.set_publications([p1.id])
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True
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>>> a2.set_publications([p1.id, p2.id, p3.id])
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True
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# Article objects have access to their related Publication objects.
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>>> a1.get_publication_list()
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[The Python Journal]
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>>> a2.get_publication_list()
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[The Python Journal, Science News, Science Weekly]
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# Publication objects have access to their related Article objects.
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>>> p2.get_article_list()
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[NASA uses Python]
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>>> p1.get_article_list(order_by=['headline'])
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[Django lets you build Web apps easily, NASA uses Python]
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# We can perform kwarg queries across m2m relationships
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>>> Article.objects.get_list(publications__id__exact=1)
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[Django lets you build Web apps easily, NASA uses Python]
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>>> Article.objects.get_list(publications__pk=1)
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[Django lets you build Web apps easily, NASA uses Python]
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>>> Article.objects.get_list(publications__title__startswith="Science")
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[NASA uses Python, NASA uses Python]
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>>> Article.objects.get_list(publications__title__startswith="Science", distinct=True)
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[NASA uses Python]
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# Reverse m2m queries (i.e., start at the table that doesn't have a ManyToManyField)
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>>> Publication.objects.get_list(id__exact=1)
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[The Python Journal]
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>>> Publication.objects.get_list(pk=1)
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[The Python Journal]
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>>> Publication.objects.get_list(articles__headline__startswith="NASA")
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[The Python Journal, Science News, Science Weekly]
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>>> Publication.objects.get_list(articles__id__exact=1)
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[The Python Journal]
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>>> Publication.objects.get_list(articles__pk=1)
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[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.get_list()
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[Science News, Science Weekly]
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>>> a1 = Article.objects.get_object(pk=1)
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>>> a1.get_publication_list()
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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.get_list()
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[Django lets you build Web apps easily]
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>>> p1.get_article_list(order_by=['headline'])
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[Django lets you build Web apps easily]
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"""
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