########################
One-to-one relationships
########################

To define a one-to-one relationship, use :ref:`ref-onetoone`.

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'.

.. versionchanged:: 1.8.4

    Previously, saving an object with unsaved related objects did not raise an
    error and could result in silent data loss. In 1.8-1.8.3, unsaved model
    instances couldn't be assigned to related fields, but this restriction was
    removed to allow easier usage of in-memory models.

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=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>]