# Django 5.2.8 documentation

# 一对一关联

要定义一对一关联，使用 `OneToOneField`。

在本例中，一个 `Place` 可是一个 `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 f"{self.name} the place"

    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)

下面是可以使用PythonAPI工具执行的操作示例。

创建几个 `Places`：

    >>> p1 = Place(name="Demon Dogs", address="944 W. Fullerton")
    >>> p1.save()
    >>> p2 = Place(name="Ace Hardware", address="1013 N. Ashland")
    >>> p2.save()

创建一个餐厅。将“parent”对象作为此对象的主键传递：

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

一个餐厅可以访问它的位置：

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

一个地点可以访问它的餐厅，如果有的话：

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

p2 没有关联的餐厅：

    >>> from django.core.exceptions import ObjectDoesNotExist
    >>> try:
    ...     p2.restaurant
    ... except ObjectDoesNotExist:
    ...     print("There is no restaurant here.")
    ...
    There is no restaurant here.

您还可以使用 `hasattr` 来避免需要捕获异常：

    >>> hasattr(p2, "restaurant")
    False

使用赋值符号设置地点。由于地点是餐厅的主键，保存操作将创建一个新的餐厅：

    >>> r.place = p2
    >>> r.save()
    >>> p2.restaurant
    <Restaurant: Ace Hardware the restaurant>
    >>> r.place
    <Place: Ace Hardware the place>

再次使用反向赋值方式设置地点：

    >>> p1.restaurant = r
    >>> p1.restaurant
    <Restaurant: Demon Dogs the restaurant>

请注意，在将对象分配给一对一关系之前，您必须先保存该对象。例如，使用未保存的 `Place` 创建一个 `Restaurant` 会引发 `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() 返回的是餐厅，而不是地点。请注意，有两家餐厅 - Ace Hardware 餐厅是在调用 r.place = p2 时创建的：

    >>> Restaurant.objects.all()
    <QuerySet [<Restaurant: Demon Dogs the restaurant>, <Restaurant: Ace Hardware the restaurant>]>

Place.objects.all() 返回所有地点，不管它们是否有餐厅：

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

您可以使用 跨关系的查找 来查询模型：

    >>> 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>]>

这也可以反过来做：

    >>> 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>

如果你删除一个地方，它的餐馆将被删除（假设 `OneToOneField` 是用 `on_delete` 设置为 `CASCADE` 定义的，这是默认值）：

    >>> p2.delete()
    (2, {'one_to_one.Restaurant': 1, 'one_to_one.Place': 1})
    >>> Restaurant.objects.all()
    <QuerySet [<Restaurant: Demon Dogs the restaurant>]>

在餐馆中添加一个服务员：

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

查询服务员：

    >>> 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>]>