# Django 1.8.18.dev20170123152445 documentation

# 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, 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)
        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 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'.

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 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>]