"""

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

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.

"""

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

__test__ = {'API_TESTS':"""

>>> 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>]

"""}