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========================
One-to-one relationships
========================
To define a one-to-one relationship, use
:class:`~django.db.models.OneToOneField`.
In this example, a ``Place`` optionally can be a ``Restaurant``::
from django.db import models
class Place(models.Model):
name = models.CharField(max_length=50)
address = models.CharField(max_length=80)
def __str__(self): # __unicode__ on Python 2
return "%s the place" % self.name
class Restaurant(models.Model):
place = models.OneToOneField(
Place,
on_delete=models.CASCADE,
primary_key=True,
)
serves_hot_dogs = models.BooleanField(default=False)
serves_pizza = models.BooleanField(default=False)
def __str__(self): # __unicode__ on Python 2
return "%s the restaurant" % self.place.name
class Waiter(models.Model):
restaurant = models.ForeignKey(Restaurant, on_delete=models.CASCADE)
name = models.CharField(max_length=50)
def __str__(self): # __unicode__ on Python 2
return "%s the waiter at %s" % (self.name, self.restaurant)
What follows are examples of operations that can be performed using the Python
API facilities.
.. highlight:: pycon
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::
>>> from django.core.exceptions import ObjectDoesNotExist
>>> try:
>>> p2.restaurant
>>> except ObjectDoesNotExist:
>>> print("There is no restaurant here.")
There is no restaurant here.
You can also use ``hasattr`` to avoid the need for exception catching::
>>> hasattr(p2, 'restaurant')
False
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>
Note that you must save an object before it can be assigned to a one-to-one
relationship. For example, creating a ``Restaurant`` with unsaved ``Place``
raises ``ValueError``::
>>> p3 = Place(name='Demon Dogs', address='944 W. Fullerton')
>>> Restaurant.objects.create(place=p3, serves_hot_dogs=True, serves_pizza=False)
Traceback (most recent call last):
...
ValueError: save() prohibited to prevent data loss due to unsaved related object 'place'.
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()
<QuerySet [<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')
<QuerySet [<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")
<QuerySet [<Restaurant: Demon Dogs the restaurant>]>
>>> Restaurant.objects.exclude(place__address__contains="Ashland")
<QuerySet [<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=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
<Waiter: Joe the waiter at Demon Dogs the restaurant>
Query the waiters::
>>> Waiter.objects.filter(restaurant__place=p1)
<QuerySet [<Waiter: Joe the waiter at Demon Dogs the restaurant>]>
>>> Waiter.objects.filter(restaurant__place__name__startswith="Demon")
<QuerySet [<Waiter: Joe the waiter at Demon Dogs the restaurant>]>
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