========================

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):

            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):

            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):

            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 "parent" object as this object's primary key::

    >>> 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() 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 also 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>]>