Django4.0 模型關聯(lián)-一對一關聯(lián)

要定義一對一關聯(lián)，使用 ?OneToOneField?。

在本例中，一個 ?Place ?是一個 ?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工具執(zhí)行的操作示例。

創(chuàng)建幾個 ?Place?：

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

創(chuàng)建一個 ?Restaurant?。傳遞父對象作為該對象的主鍵：

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

?Restaurant?可以獲取所在地

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

?Place?可以訪問關聯(lián)的?Restaurant?（如果有的話）：

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

p2 沒有關聯(lián)餐廳：

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

您還可以使用 ?hasattr ?來免除異常捕獲：

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

使用賦值符號來設置?Place?。因為?Place?是?Restaurant?的主鍵，保存將創(chuàng)建一個新的?Restaurant?：

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

再次設置?Place?，使用相反方向的賦值：

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

注意，將某個對象指定給一個一對一關聯(lián)關系之前，必須先保存它。例如，利用未保存的 ?Place ?創(chuàng)建一個 ?Restaurant ?會拋出 ?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()? 返回所有?Restaurant?，而不是?Place?。注意，這里有兩個?Restaurant ?—— ?Ace Hardware the Restaurant? 是在調(diào)用 ?r.place = p2? 時創(chuàng)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?，不管其是否有關聯(lián)?Restaurant?:

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

你可以用 跨關聯(lián)查詢 查詢這些模型:

>>> 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>]>

反向查詢也是可以的:

>>> 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>

為?Restaurant?添加一個?Waiter?:

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

查詢?Waiter?:

>>> 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>]>