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Fixed #17605: Restored deleted query documentation that used to live in doctests. Thanks zsiciarz for work on the patch. 

git-svn-id: http://code.djangoproject.com/svn/django/trunk@17737 bcc190cf-cafb-0310-a4f2-bffc1f526a37
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James Bennett 2012-03-14 22:16:46 +00:00
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========================================
Examples of model relationship API usage
========================================
.. toctree::
:maxdepth: 1
many_to_many
many_to_one
one_to_one

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##########################
Many-to-many relationships
##########################
.. highlight:: pycon
To define a many-to-many relationship, use :ref:`ref-manytomany`.
In this example, an ``Article`` can be published in multiple ``Publication``
objects, and a ``Publication`` has multiple ``Article`` objects:
.. code-block:: python
from django.db import models
class Publication(models.Model):
title = models.CharField(max_length=30)
def __unicode__(self):
return self.title
class Meta:
ordering = ('title',)
class Article(models.Model):
headline = models.CharField(max_length=100)
publications = models.ManyToManyField(Publication)
def __unicode__(self):
return self.headline
class Meta:
ordering = ('headline',)
What follows are examples of operations that can be performed using the Python
API facilities.
Create a couple of Publications::
>>> p1 = Publication(title='The Python Journal')
>>> p1.save()
>>> p2 = Publication(title='Science News')
>>> p2.save()
>>> p3 = Publication(title='Science Weekly')
>>> p3.save()
Create an Article::
>>> a1 = Article(headline='Django lets you build Web apps easily')
You can't associate it with a Publication until it's been saved::
>>> a1.publications.add(p1)
Traceback (most recent call last):
...
ValueError: 'Article' instance needs to have a primary key value before a many-to-many relationship can be used.
Save it!
::
>>> 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(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)
Adding an object of the wrong type raises TypeError::
>>> a2.publications.add(a1)
Traceback (most recent call last):
...
TypeError: 'Publication' instance expected
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>]
Many-to-many relationships can be queried using :ref:`lookups across relationships <lookups-that-span-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=[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=[a1,a2]).distinct()
[<Publication: Highlights for Children>, <Publication: Science News>, <Publication: Science Weekly>, <Publication: The Python Journal>]
Excluding a related item works as you would expect, too (although the SQL
involved is a little complex)::
>>> Article.objects.exclude(publications=p2)
[<Article: Django lets you build Web apps easily>]
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::
>>> a4.publications.all()
[<Publication: Science News>]
>>> a4.publications = [p3]
>>> a4.publications.all()
[<Publication: Science Weekly>]
Relation sets can be cleared::
>>> p2.article_set.clear()
>>> p2.article_set.all()
[]
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>]
Recreate the article and Publication we have deleted::
>>> p1 = Publication(title='The Python Journal')
>>> p1.save()
>>> a2 = Article(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()
[]

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#########################
Many-to-one relationships
#########################
.. highlight:: pycon
To define a many-to-one relationship, use :class:`~django.db.models.ForeignKey`.
.. code-block:: python
from django.db import models
class Reporter(models.Model):
first_name = models.CharField(max_length=30)
last_name = models.CharField(max_length=30)
email = models.EmailField()
def __unicode__(self):
return u"%s %s" % (self.first_name, self.last_name)
class Article(models.Model):
headline = models.CharField(max_length=100)
pub_date = models.DateField()
reporter = models.ForeignKey(Reporter)
def __unicode__(self):
return self.headline
class Meta:
ordering = ('headline',)
What follows are examples of operations that can be performed using the Python
API facilities.
Create a few Reporters::
>>> r = Reporter(first_name='John', last_name='Smith', email='john@example.com')
>>> r.save()
>>> r2 = Reporter(first_name='Paul', last_name='Jones', email='paul@example.com')
>>> r2.save()
Create an Article::
>>> from datetime import datetime
>>> a = Article(id=None, headline="This is a test", pub_date=datetime(2005, 7, 27), reporter=r)
>>> a.save()
>>> a.reporter.id
1
>>> a.reporter
<Reporter: John Smith>
Article objects have access to their related Reporter objects::
>>> r = a.reporter
These are strings instead of unicode strings because that's what was used in
the creation of this reporter (and we haven't refreshed the data from the
database, which always returns unicode strings)::
>>> r.first_name, r.last_name
('John', 'Smith')
Create an Article via the Reporter object::
>>> new_article = r.article_set.create(headline="John's second story", pub_date=datetime(2005, 7, 29))
>>> new_article
<Article: John's second story>
>>> new_article.reporter
<Reporter: John Smith>
>>> new_article.reporter.id
1
Create a new article, and add it to the article set::
>>> new_article2 = Article(headline="Paul's story", pub_date=datetime(2006, 1, 17))
>>> r.article_set.add(new_article2)
>>> new_article2.reporter
<Reporter: John Smith>
>>> new_article2.reporter.id
1
>>> r.article_set.all()
[<Article: John's second story>, <Article: Paul's story>, <Article: This is a test>]
Add the same article to a different article set - check that it moves::
>>> r2.article_set.add(new_article2)
>>> new_article2.reporter.id
2
>>> new_article2.reporter
<Reporter: Paul Jones>
Adding an object of the wrong type raises TypeError::
>>> r.article_set.add(r2)
Traceback (most recent call last):
...
TypeError: 'Article' instance expected
>>> r.article_set.all()
[<Article: John's second story>, <Article: This is a test>]
>>> r2.article_set.all()
[<Article: Paul's story>]
>>> r.article_set.count()
2
>>> r2.article_set.count()
1
Note that in the last example the article has moved from John to Paul.
Related managers support field lookups as well.
The API automatically follows relationships as far as you need.
Use double underscores to separate relationships.
This works as many levels deep as you want. There's no limit. For example::
>>> r.article_set.filter(headline__startswith='This')
[<Article: This is a test>]
# Find all Articles for any Reporter whose first name is "John".
>>> Article.objects.filter(reporter__first_name__exact='John')
[<Article: John's second story>, <Article: This is a test>]
Exact match is implied here::
>>> Article.objects.filter(reporter__first_name='John')
[<Article: John's second story>, <Article: This is a test>]
Query twice over the related field. This translates to an AND condition in the
WHERE clause::
>>> Article.objects.filter(reporter__first_name__exact='John', reporter__last_name__exact='Smith')
[<Article: John's second story>, <Article: This is a test>]
For the related lookup you can supply a primary key value or pass the related
object explicitly::
>>> Article.objects.filter(reporter__pk=1)
[<Article: John's second story>, <Article: This is a test>]
>>> Article.objects.filter(reporter=1)
[<Article: John's second story>, <Article: This is a test>]
>>> Article.objects.filter(reporter=r)
[<Article: John's second story>, <Article: This is a test>]
>>> Article.objects.filter(reporter__in=[1,2]).distinct()
[<Article: John's second story>, <Article: Paul's story>, <Article: This is a test>]
>>> Article.objects.filter(reporter__in=[r,r2]).distinct()
[<Article: John's second story>, <Article: Paul's story>, <Article: This is a test>]
You can also use a queryset instead of a literal list of instances::
>>> Article.objects.filter(reporter__in=Reporter.objects.filter(first_name='John')).distinct()
[<Article: John's second story>, <Article: This is a test>]
Querying in the opposite direction::
>>> Reporter.objects.filter(article__pk=1)
[<Reporter: John Smith>]
>>> Reporter.objects.filter(article=1)
[<Reporter: John Smith>]
>>> Reporter.objects.filter(article=a)
[<Reporter: John Smith>]
>>> Reporter.objects.filter(article__headline__startswith='This')
[<Reporter: John Smith>, <Reporter: John Smith>, <Reporter: John Smith>]
>>> Reporter.objects.filter(article__headline__startswith='This').distinct()
[<Reporter: John Smith>]
Counting in the opposite direction works in conjunction with distinct()::
>>> Reporter.objects.filter(article__headline__startswith='This').count()
3
>>> Reporter.objects.filter(article__headline__startswith='This').distinct().count()
1
Queries can go round in circles::
>>> Reporter.objects.filter(article__reporter__first_name__startswith='John')
[<Reporter: John Smith>, <Reporter: John Smith>, <Reporter: John Smith>, <Reporter: John Smith>]
>>> Reporter.objects.filter(article__reporter__first_name__startswith='John').distinct()
[<Reporter: John Smith>]
>>> Reporter.objects.filter(article__reporter__exact=r).distinct()
[<Reporter: John Smith>]
If you delete a reporter, his articles will be deleted (assuming that the
ForeignKey was defined with :attr:`django.db.models.ForeignKey.on_delete` set to
``CASCADE``, which is the default)::
>>> Article.objects.all()
[<Article: John's second story>, <Article: Paul's story>, <Article: This is a test>]
>>> Reporter.objects.order_by('first_name')
[<Reporter: John Smith>, <Reporter: Paul Jones>]
>>> r2.delete()
>>> Article.objects.all()
[<Article: John's second story>, <Article: This is a test>]
>>> Reporter.objects.order_by('first_name')
[<Reporter: John Smith>]
You can delete using a JOIN in the query::
>>> Reporter.objects.filter(article__headline__startswith='This').delete()
>>> Reporter.objects.all()
[]
>>> Article.objects.all()
[]

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########################
One-to-one relationships
########################
.. highlight:: pycon
To define a one-to-one relationship, use :ref:`ref-onetoone`.
In this example, a ``Place`` optionally can be a ``Restaurant``:
.. code-block:: python
from django.db import models, transaction, IntegrityError
class Place(models.Model):
name = models.CharField(max_length=50)
address = models.CharField(max_length=80)
def __unicode__(self):
return u"%s the place" % self.name
class Restaurant(models.Model):
place = models.OneToOneField(Place, primary_key=True)
serves_hot_dogs = models.BooleanField()
serves_pizza = models.BooleanField()
def __unicode__(self):
return u"%s the restaurant" % self.place.name
class Waiter(models.Model):
restaurant = models.ForeignKey(Restaurant)
name = models.CharField(max_length=50)
def __unicode__(self):
return u"%s the waiter at %s" % (self.name, self.restaurant)
What follows are examples of operations that can be performed using the Python
API facilities.
Create a couple of Places::
>>> p1 = Place(name='Demon Dogs', address='944 W. Fullerton')
>>> p1.save()
>>> p2 = Place(name='Ace Hardware', address='1013 N. Ashland')
>>> p2.save()
Create a Restaurant. Pass the ID of the "parent" object as this object's ID::
>>> r = Restaurant(place=p1, serves_hot_dogs=True, serves_pizza=False)
>>> r.save()
A Restaurant can access its place::
>>> r.place
<Place: Demon Dogs the place>
A Place can access its restaurant, if available::
>>> p1.restaurant
<Restaurant: Demon Dogs the restaurant>
p2 doesn't have an associated restaurant::
>>> p2.restaurant
Traceback (most recent call last):
...
DoesNotExist: Restaurant matching query does not exist.
Set the place using assignment notation. Because place is the primary key on
Restaurant, the save will create a new restaurant::
>>> r.place = p2
>>> r.save()
>>> p2.restaurant
<Restaurant: Ace Hardware the restaurant>
>>> r.place
<Place: Ace Hardware the place>
Set the place back again, using assignment in the reverse direction::
>>> p1.restaurant = r
>>> p1.restaurant
<Restaurant: Demon Dogs the restaurant>
Restaurant.objects.all() just returns the Restaurants, not the Places. Note
that there are two restaurants - Ace Hardware the Restaurant was created in the
call to r.place = p2::
>>> Restaurant.objects.all()
[<Restaurant: Demon Dogs the restaurant>, <Restaurant: Ace Hardware the restaurant>]
Place.objects.all() returns all Places, regardless of whether they have
Restaurants::
>>> Place.objects.order_by('name')
[<Place: Ace Hardware the place>, <Place: Demon Dogs the place>]
You can query the models using :ref:`lookups across relationships <lookups-that-span-relationships>`::
>>> Restaurant.objects.get(place=p1)
<Restaurant: Demon Dogs the restaurant>
>>> Restaurant.objects.get(place__pk=1)
<Restaurant: Demon Dogs the restaurant>
>>> Restaurant.objects.filter(place__name__startswith="Demon")
[<Restaurant: Demon Dogs the restaurant>]
>>> Restaurant.objects.exclude(place__address__contains="Ashland")
[<Restaurant: Demon Dogs the restaurant>]
This of course works in reverse::
>>> Place.objects.get(pk=1)
<Place: Demon Dogs the place>
>>> Place.objects.get(restaurant__place__exact=p1)
<Place: Demon Dogs the place>
>>> Place.objects.get(restaurant=r)
<Place: Demon Dogs the place>
>>> Place.objects.get(restaurant__place__name__startswith="Demon")
<Place: Demon Dogs the place>
Add a Waiter to the Restaurant::
>>> w = r.waiter_set.create(name='Joe')
>>> w.save()
>>> w
<Waiter: Joe the waiter at Demon Dogs the restaurant>
Query the waiters::
>>> Waiter.objects.filter(restaurant__place=p1)
[<Waiter: Joe the waiter at Demon Dogs the restaurant>]
>>> Waiter.objects.filter(restaurant__place__name__startswith="Demon")
[<Waiter: Joe the waiter at Demon Dogs the restaurant>]

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@ -19,3 +19,4 @@ model maps to a single database table.
multi-db
tablespaces
optimization
examples/index

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@ -18,13 +18,6 @@ The basics:
* With all of this, Django gives you an automatically-generated
database-access API; see :doc:`/topics/db/queries`.
.. seealso::
A companion to this document is the `official repository of model
examples`_. (In the Django source distribution, these examples are in the
``tests/modeltests`` directory.)
.. _official repository of model examples: https://code.djangoproject.com/browser/django/trunk/tests/modeltests
Quick example
=============
@ -326,9 +319,6 @@ whatever you want. For example::
For details on accessing backwards-related objects, see the
:ref:`Following relationships backward example <backwards-related-objects>`.
For sample code, see the `Many-to-one relationship model tests`_.
.. _Many-to-one relationship model tests: https://code.djangoproject.com/browser/django/trunk/tests/modeltests/many_to_one
Many-to-many relationships
~~~~~~~~~~~~~~~~~~~~~~~~~~
@ -376,10 +366,6 @@ form would let users select the toppings.
.. seealso::
See the `Many-to-many relationship model example`_ for a full example.
.. _Many-to-many relationship model example: https://code.djangoproject.com/browser/django/trunk/tests/modeltests/many_to_many/models.py
:class:`~django.db.models.ManyToManyField` fields also accept a number of extra
arguments which are explained in :ref:`the model field reference
<manytomany-arguments>`. These options help define how the relationship should
@ -569,10 +555,6 @@ can be made; see :ref:`the model field reference <ref-onetoone>` for details.
.. seealso::
See the `One-to-one relationship model example`_ for a full example.
.. _One-to-one relationship model example: https://code.djangoproject.com/browser/django/trunk/tests/modeltests/one_to_one/models.py
:class:`~django.db.models.OneToOneField` fields also accept one optional argument
described in the :ref:`model field reference <ref-onetoone>`.

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@ -483,6 +483,8 @@ probably use:
Again, this only scratches the surface. A complete reference can be found in the
:ref:`field lookup reference <field-lookups>`.
.. _lookups-that-span-relationships:
Lookups that span relationships
-------------------------------