Django documentation

20. Multiple many-to-many relationships between the same two tables

This example describes Django version 0.96. For the current example, go here.

In this example, an Article can have many Categories (as “primary”) and many Categories (as “secondary”).

Set related_name to designate what the reverse relationship is called.

Model source code

from django.db import models

class Category(models.Model):
    name = models.CharField(maxlength=20)
    class Meta:
       ordering = ('name',)

    def __str__(self):
        return self.name

class Article(models.Model):
    headline = models.CharField(maxlength=50)
    pub_date = models.DateTimeField()
    primary_categories = models.ManyToManyField(Category, related_name='primary_article_set')
    secondary_categories = models.ManyToManyField(Category, related_name='secondary_article_set')
    class Meta:
       ordering = ('pub_date',)

    def __str__(self):
        return self.headline

Sample API usage

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

>>> from mysite.models import Category, Article

>>> from datetime import datetime

>>> c1 = Category(name='Sports')
>>> c1.save()
>>> c2 = Category(name='News')
>>> c2.save()
>>> c3 = Category(name='Crime')
>>> c3.save()
>>> c4 = Category(name='Life')
>>> c4.save()

>>> a1 = Article(headline='Area man steals', pub_date=datetime(2005, 11, 27))
>>> a1.save()
>>> a1.primary_categories.add(c2, c3)
>>> a1.secondary_categories.add(c4)

>>> a2 = Article(headline='Area man runs', pub_date=datetime(2005, 11, 28))
>>> a2.save()
>>> a2.primary_categories.add(c1, c2)
>>> a2.secondary_categories.add(c4)

>>> a1.primary_categories.all()
[<Category: Crime>, <Category: News>]

>>> a2.primary_categories.all()
[<Category: News>, <Category: Sports>]

>>> a1.secondary_categories.all()
[<Category: Life>]


>>> c1.primary_article_set.all()
[<Article: Area man runs>]
>>> c1.secondary_article_set.all()
[]
>>> c2.primary_article_set.all()
[<Article: Area man steals>, <Article: Area man runs>]
>>> c2.secondary_article_set.all()
[]
>>> c3.primary_article_set.all()
[<Article: Area man steals>]
>>> c3.secondary_article_set.all()
[]
>>> c4.primary_article_set.all()
[]
>>> c4.secondary_article_set.all()
[<Article: Area man steals>, <Article: Area man runs>]

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