# Django 2.2.9.dev20191202104323 文档

# 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 "%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)

下面是可以使用PythonAPI工具执行的操作示例。

创建几个 Place：

    >>> 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 Restaurant。将 “父” 对象的 ID 传递为该对象的 ID：

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

餐厅可以获取所在地：

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

地点可以访问关联的餐厅（如果有的话）：

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

p2 没有关联餐厅：

    >>> 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` 来免除异常捕获：

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

使用赋值符号来设置地方。因为地方是餐厅的主键，保存将创建一个新的餐馆：

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

再次设置地方，使用相反方向的赋值：

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

注意，将某个对象指定给一个一对一关联关系之前，必须先保存它。例如，利用未保存的 `Place` 创建一个 `Restaurant` 会抛出 `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() 仅返回所有餐厅，而不是地点。注意，这里有两个餐厅 —— Ace Hardware the Restaurant 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() 返回所有的 Place，不管其是否有关联餐厅:

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

你可以用 跨关联查询 查询这些模型:

    >>> 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>]>

反向查询当然也是可以的:

    >>> 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>

为餐厅添加一个服务员:

    >>> 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>]>