One-to-one relationships

To define a one-to-one relationship, use 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.

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 an Restaurant with unsaved Place raises ValueError:

>>> p3 = Place(name='Demon Dogs', address='944 W. Fullerton')
>>> Restaurant(place=p3, serves_hot_dogs=True, serves_pizza=False)
Traceback (most recent call last):
...
ValueError: 'Cannot assign "<Place: Demon Dogs>": "Place" instance isn't saved in the database.'
>>> p.restaurant = Restaurant(place=p, serves_hot_dogs=True, serves_pizza=False)
Traceback (most recent call last):
...
ValueError: 'Cannot assign "<Restaurant: Demon Dogs the restaurant>": "Restaurant" instance isn't saved in the database.'

If you want to disable the unsaved instance check, you can use the allow_unsaved_instance_assignment attribute.

Changed in Django 1.8:

Previously, assigning unsaved objects did not raise an error and could result in silent data loss.

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 lookups across 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=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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