要定义一对一关联,使用 OneToOneField
。
在本例中,一个 Place
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
。传递父对象作为该对象的主键:
>>> r = Restaurant(place=p1, serves_hot_dogs=True, serves_pizza=False)
>>> r.save()
Restaurant
可以获取所在地
>>> r.place
<Place: Demon Dogs the place>
Place
可以访问关联的Restaurant
(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Place
。因为Place
是Restaurant
的主键,保存将创建一个新的Restaurant
:
>>> r.place = p2
>>> r.save()
>>> p2.restaurant
<Restaurant: Ace Hardware the restaurant>
>>> r.place
<Place: Ace Hardware the place>
再次设置Place
,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()
返回所有Restaurant
,而不是Place
。注意,这里有两个Restaurant
—— 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Restaurant
:
>>> 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Restaurant
添加一个Waiter
:
>>> w = r.waiter_set.create(name="Joe")
>>> w
<Waiter: Joe the waiter at Demon Dogs the restaurant>
查询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>]>
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