"""

5. Many-to-many relationships

To define a many-to-many relationship, use ManyToManyField().

In this example, an article can be published in multiple publications,

and a publication has multiple articles.

"""

from django.db import models

class Publication(models.Model):

    title = models.CharField(maxlength=30)

    def __str__(self):

        return self.title

    class Meta:

        ordering = ('title',)

class Article(models.Model):

    headline = models.CharField(maxlength=100)

    publications = models.ManyToManyField(Publication)

    def __str__(self):

        return self.headline

    class Meta:

        ordering = ('headline',)

__test__ = {'API_TESTS':"""

# Create a couple of Publications.

>>> p1 = Publication(id=None, title='The Python Journal')

>>> p1.save()

>>> p2 = Publication(id=None, title='Science News')

>>> p2.save()

>>> p3 = Publication(id=None, title='Science Weekly')

>>> p3.save()

# Create an Article.

>>> a1 = Article(id=None, headline='Django lets you build Web apps easily')

>>> a1.save()

# Associate the Article with a Publication.

>>> a1.publications.add(p1)

# Create another Article, and set it to appear in both Publications.

>>> a2 = Article(id=None, headline='NASA uses Python')

>>> a2.save()

>>> a2.publications.add(p1, p2)

>>> a2.publications.add(p3)

# Adding a second time is OK

>>> a2.publications.add(p3)

# Add a Publication directly via publications.add by using keyword arguments.

>>> new_publication = a2.publications.create(title='Highlights for Children')

# Article objects have access to their related Publication objects.

>>> a1.publications.all()

[<Publication: The Python Journal>]

>>> a2.publications.all()

[<Publication: Highlights for Children>, <Publication: Science News>, <Publication: Science Weekly>, <Publication: The Python Journal>]

# Publication objects have access to their related Article objects.

>>> p2.article_set.all()

[<Article: NASA uses Python>]

>>> p1.article_set.all()

[<Article: Django lets you build Web apps easily>, <Article: NASA uses Python>]

>>> Publication.objects.get(id=4).article_set.all()

[<Article: NASA uses Python>]

# We can perform kwarg queries across m2m relationships

>>> Article.objects.filter(publications__id__exact=1)

[<Article: Django lets you build Web apps easily>, <Article: NASA uses Python>]

>>> Article.objects.filter(publications__pk=1)

[<Article: Django lets you build Web apps easily>, <Article: NASA uses Python>]

>>> Article.objects.filter(publications=1)

[<Article: Django lets you build Web apps easily>, <Article: NASA uses Python>]

>>> Article.objects.filter(publications=p1)

[<Article: Django lets you build Web apps easily>, <Article: NASA uses Python>]

>>> Article.objects.filter(publications__title__startswith="Science")

[<Article: NASA uses Python>, <Article: NASA uses Python>]

>>> Article.objects.filter(publications__title__startswith="Science").distinct()

[<Article: NASA uses Python>]

# The count() function respects distinct() as well.

>>> Article.objects.filter(publications__title__startswith="Science").count()

2

>>> Article.objects.filter(publications__title__startswith="Science").distinct().count()

1

>>> Article.objects.filter(publications__in=[1,2]).distinct()

[<Article: Django lets you build Web apps easily>, <Article: NASA uses Python>]

>>> Article.objects.filter(publications__in=[1,p2]).distinct()

[<Article: Django lets you build Web apps easily>, <Article: NASA uses Python>]

>>> Article.objects.filter(publications__in=[p1,p2]).distinct()

[<Article: Django lets you build Web apps easily>, <Article: NASA uses Python>]

# Reverse m2m queries are supported (i.e., starting at the table that doesn't

# have a ManyToManyField).

>>> Publication.objects.filter(id__exact=1)

[<Publication: The Python Journal>]

>>> Publication.objects.filter(pk=1)

[<Publication: The Python Journal>]

>>> Publication.objects.filter(article__headline__startswith="NASA")

[<Publication: Highlights for Children>, <Publication: Science News>, <Publication: Science Weekly>, <Publication: The Python Journal>]

>>> Publication.objects.filter(article__id__exact=1)

[<Publication: The Python Journal>]

>>> Publication.objects.filter(article__pk=1)

[<Publication: The Python Journal>]

>>> Publication.objects.filter(article=1)

[<Publication: The Python Journal>]

>>> Publication.objects.filter(article=a1)

[<Publication: The Python Journal>]

>>> Publication.objects.filter(article__in=[1,2]).distinct()

[<Publication: Highlights for Children>, <Publication: Science News>, <Publication: Science Weekly>, <Publication: The Python Journal>]

>>> Publication.objects.filter(article__in=[1,a2]).distinct()

[<Publication: Highlights for Children>, <Publication: Science News>, <Publication: Science Weekly>, <Publication: The Python Journal>]

>>> Publication.objects.filter(article__in=[a1,a2]).distinct()

[<Publication: Highlights for Children>, <Publication: Science News>, <Publication: Science Weekly>, <Publication: The Python Journal>]

# If we delete a Publication, its Articles won't be able to access it.

>>> p1.delete()

>>> Publication.objects.all()

[<Publication: Highlights for Children>, <Publication: Science News>, <Publication: Science Weekly>]

>>> a1 = Article.objects.get(pk=1)

>>> a1.publications.all()

[]

# If we delete an Article, its Publications won't be able to access it.

>>> a2.delete()

>>> Article.objects.all()

[<Article: Django lets you build Web apps easily>]

>>> p2.article_set.all()

[]

# Adding via the 'other' end of an m2m

>>> a4 = Article(headline='NASA finds intelligent life on Earth')

>>> a4.save()

>>> p2.article_set.add(a4)

>>> p2.article_set.all()

[<Article: NASA finds intelligent life on Earth>]

>>> a4.publications.all()

[<Publication: Science News>]

# Adding via the other end using keywords

>>> new_article = p2.article_set.create(headline='Oxygen-free diet works wonders')

>>> p2.article_set.all()

[<Article: NASA finds intelligent life on Earth>, <Article: Oxygen-free diet works wonders>]

>>> a5 = p2.article_set.all()[1]

>>> a5.publications.all()

[<Publication: Science News>]

# Removing publication from an article:

>>> a4.publications.remove(p2)

>>> p2.article_set.all()

[<Article: Oxygen-free diet works wonders>]

>>> a4.publications.all()

[]

# And from the other end

>>> p2.article_set.remove(a5)

>>> p2.article_set.all()

[]

>>> a5.publications.all()

[]

# Relation sets can be assigned. Assignment clears any existing set members

>>> p2.article_set = [a4, a5]

>>> p2.article_set.all()

[<Article: NASA finds intelligent life on Earth>, <Article: Oxygen-free diet works wonders>]

>>> a4.publications.all()

[<Publication: Science News>]

>>> a4.publications = [p3]

>>> p2.article_set.all()

[<Article: Oxygen-free diet works wonders>]

>>> a4.publications.all()

[<Publication: Science Weekly>]

# Relation sets can be cleared:

>>> p2.article_set.clear()

>>> p2.article_set.all()

[]

>>> a4.publications.all()

[<Publication: Science Weekly>]

# And you can clear from the other end

>>> p2.article_set.add(a4, a5)

>>> p2.article_set.all()

[<Article: NASA finds intelligent life on Earth>, <Article: Oxygen-free diet works wonders>]

>>> a4.publications.all()

[<Publication: Science News>, <Publication: Science Weekly>]

>>> a4.publications.clear()

>>> a4.publications.all()

[]

>>> p2.article_set.all()

[<Article: Oxygen-free diet works wonders>]

# Relation sets can also be set using primary key values

>>> p2.article_set = [a4.id, a5.id]

>>> p2.article_set.all()

[<Article: NASA finds intelligent life on Earth>, <Article: Oxygen-free diet works wonders>]

>>> a4.publications.all()

[<Publication: Science News>]

>>> a4.publications = [p3.id]

>>> p2.article_set.all()

[<Article: Oxygen-free diet works wonders>]

>>> a4.publications.all()

[<Publication: Science Weekly>]

# Recreate the article and Publication we have deleted.

>>> p1 = Publication(id=None, title='The Python Journal')

>>> p1.save()

>>> a2 = Article(id=None, headline='NASA uses Python')

>>> a2.save()

>>> a2.publications.add(p1, p2, p3)

# Bulk delete some Publications - references to deleted publications should go

>>> Publication.objects.filter(title__startswith='Science').delete()

>>> Publication.objects.all()

[<Publication: Highlights for Children>, <Publication: The Python Journal>]

>>> Article.objects.all()

[<Article: Django lets you build Web apps easily>, <Article: NASA finds intelligent life on Earth>, <Article: NASA uses Python>, <Article: Oxygen-free diet works wonders>]

>>> a2.publications.all()

[<Publication: The Python Journal>]

# Bulk delete some articles - references to deleted objects should go

>>> q = Article.objects.filter(headline__startswith='Django')

>>> print q

[<Article: Django lets you build Web apps easily>]

>>> q.delete()

# After the delete, the QuerySet cache needs to be cleared, and the referenced objects should be gone

>>> print q

[]

>>> p1.article_set.all()

[<Article: NASA uses Python>]

# An alternate to calling clear() is to assign the empty set

>>> p1.article_set = []

>>> p1.article_set.all()

[]

>>> a2.publications = [p1, new_publication]

>>> a2.publications.all()

[<Publication: Highlights for Children>, <Publication: The Python Journal>]

>>> a2.publications = []

>>> a2.publications.all()

[]

"""}