一對一關係¶
若要定義一對一關係，請使用 OneToOneField。
在這個範例中，Place（地點）可以選擇性地是一個 Restaurant（餐廳）
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)
以下是可以使用 Python API 設施執行的操作範例。
建立幾個 Place（地點）
>>> p1 = Place(name="Demon Dogs", address="944 W. Fullerton")
>>> p1.save()
>>> p2 = Place(name="Ace Hardware", address="1013 N. Ashland")
>>> p2.save()
建立一個 Restaurant（餐廳）。傳遞「父」物件作為此物件的主鍵
>>> r = Restaurant(place=p1, serves_hot_dogs=True, serves_pizza=False)
>>> r.save()
一個 Restaurant（餐廳）可以存取其 Place（地點）
>>> r.place
<Place: Demon Dogs the place>
一個 Place（地點）可以存取其 Restaurant（餐廳），如果有的話
>>> p1.restaurant
<Restaurant: Demon Dogs the restaurant>
p2 沒有關聯的餐廳
>>> from django.core.exceptions import ObjectDoesNotExist
>>> try:
... p2.restaurant
... except ObjectDoesNotExist:
... print("There is no restaurant here.")
...
There is no restaurant here.
您也可以使用 hasattr 來避免需要捕捉例外
>>> hasattr(p2, "restaurant")
False
使用賦值符號設定地點。因為地點是 Restaurant（餐廳）上的主鍵，儲存會建立一個新的餐廳
>>> r.place = p2
>>> r.save()
>>> p2.restaurant
<Restaurant: Ace Hardware the restaurant>
>>> r.place
<Place: Ace Hardware the place>
再次設定地點，使用反向的賦值
>>> p1.restaurant = r
>>> p1.restaurant
<Restaurant: Demon Dogs the restaurant>
請注意，您必須先儲存一個物件，才能將其指派給一對一關係。例如，使用未儲存的 Place（地點）建立一個 Restaurant（餐廳）會引發 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() 回傳的是 Restaurant（餐廳），而不是 Place（地點）。請注意，有兩個餐廳 - Ace Hardware 餐廳是在呼叫 r.place = p2 時建立的
>>> Restaurant.objects.all()
<QuerySet [<Restaurant: Demon Dogs the restaurant>, <Restaurant: Ace Hardware the restaurant>]>
Place.objects.all() 回傳所有 Place（地點），無論它們是否有 Restaurant（餐廳）
>>> Place.objects.order_by("name")
<QuerySet [<Place: Ace Hardware the place>, <Place: Demon Dogs the place>]>
您可以使用 跨關係的查詢來查詢模型
>>> 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>]>
這也適用於反向查詢
>>> 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>
如果您刪除一個地點，它的餐廳也會被刪除（假設 OneToOneField 的 on_delete 設定為 CASCADE，這是預設值）
>>> p2.delete()
(2, {'one_to_one.Restaurant': 1, 'one_to_one.Place': 1})
>>> Restaurant.objects.all()
<QuerySet [<Restaurant: Demon Dogs the restaurant>]>
為 Restaurant（餐廳）新增一個 Waiter（服務生）
>>> w = r.waiter_set.create(name="Joe")
>>> w
<Waiter: Joe the waiter at Demon Dogs the restaurant>
查詢服務生
>>> 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>]>
