Django documentation

9. Many-to-many relationships via an intermediary table

These docs are frozen for Django version 0.91. For current documentation, go here.

For many-to-many relationships that need extra fields on the intermediary table, use an intermediary model.

In this example, an Article can have multiple Reporter``s, and each ``Article-Reporter combination (a Writer) has a position field, which specifies the Reporter's position for the given article (e.g. "Staff writer").

Model source code

from django.core import meta

class Reporter(meta.Model):
    first_name = meta.CharField(maxlength=30)
    last_name = meta.CharField(maxlength=30)

    def __repr__(self):
        return "%s %s" % (self.first_name, self.last_name)

class Article(meta.Model):
    headline = meta.CharField(maxlength=100)
    pub_date = meta.DateField()

    def __repr__(self):
        return self.headline

class Writer(meta.Model):
    reporter = meta.ForeignKey(Reporter)
    article = meta.ForeignKey(Article)
    position = meta.CharField(maxlength=100)

    def __repr__(self):
        return '%r (%s)' % (self.get_reporter(), self.position)

API reference

Reporter objects have the following methods:

  • add_writer()
  • delete()
  • get_writer()
  • get_writer_count()
  • get_writer_list()
  • save()

Article objects have the following methods:

  • add_writer()
  • delete()
  • get_next_by_pub_date()
  • get_previous_by_pub_date()
  • get_writer()
  • get_writer_count()
  • get_writer_list()
  • save()

Writer objects have the following methods:

  • delete()
  • get_article()
  • get_reporter()
  • save()

Sample API usage

This sample code assumes the above models have been saved in a file examplemodel.py. 

>>> from django.models.examplemodel import reporters, articles, writers

# Create a few Reporters.
>>> r1 = reporters.Reporter(first_name='John', last_name='Smith')
>>> r1.save()
>>> r2 = reporters.Reporter(first_name='Jane', last_name='Doe')
>>> r2.save()

# Create an Article.
>>> from datetime import datetime
>>> a = articles.Article(headline='This is a test', pub_date=datetime(2005, 7, 27))
>>> a.save()

# Create a few Writers.
>>> w1 = writers.Writer(reporter=r1, article=a, position='Main writer')
>>> w1.save()
>>> w2 = writers.Writer(reporter=r2, article=a, position='Contributor')
>>> w2.save()

# Play around with the API.
>>> a.get_writer_list(order_by=['-position'], select_related=True)
[John Smith (Main writer), Jane Doe (Contributor)]
>>> w1.get_reporter()
John Smith
>>> w2.get_reporter()
Jane Doe
>>> w1.get_article()
This is a test
>>> w2.get_article()
This is a test
>>> r1.get_writer_list()
[John Smith (Main writer)]

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