---
title: "一対一 (one-to-one) 関係"
version: 2.0
locale: ja
source: https://docs.djangoproject.com/ja/2.0/topics/db/examples/one_to_one/
canonical: https://djangodocs.dev/ja/2.0/topics/db/examples/one_to_one/
---
# 一対一 (one-to-one) 関係

To define a one-to-one relationship, use
[`OneToOneField`](/ja/2.0/ref/models/fields/#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.

Create a couple of Places:

```pycon
>>> 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:

```pycon
>>> r = Restaurant(place=p1, serves_hot_dogs=True, serves_pizza=False)
>>> r.save()
```

A Restaurant can access its place:

```pycon
>>> r.place
<Place: Demon Dogs the place>
```

A Place can access its restaurant, if available:

```pycon
>>> p1.restaurant
<Restaurant: Demon Dogs the restaurant>
```

p2 doesn't have an associated restaurant:

```pycon
>>> 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:

```pycon
>>> 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:

```pycon
>>> 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:

```pycon
>>> 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`:

```pycon
>>> 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:

```pycon
>>> 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:

```pycon
>>> Place.objects.order_by('name')
<QuerySet [<Place: Ace Hardware the place>, <Place: Demon Dogs the place>]>
```

You can query the models using [lookups across relationships](/ja/2.0/topics/db/queries/#lookups-that-span-relationships):

```pycon
>>> 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:

```pycon
>>> 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:

```pycon
>>> w = r.waiter_set.create(name='Joe')
>>> w
<Waiter: Joe the waiter at Demon Dogs the restaurant>
```

Query the waiters:

```pycon
>>> 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>]>
```