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>]>
多对一关联
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