one-to-one 关系
定义一个one-to-one关系，使用OneToOneField，用起来和其他字段差不多。该字段需要一个位置参数用来指定要关系的model.
举个简单的例子：
from django.db import models
class Place(models.Model):
name = models.CharField(max_length=50)
address = models.CharField(max_length=80)
def __str__(self): # __unicode__ on Python 2
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): # __unicode__ on Python 2
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): # __unicode__ on Python 2
return "%s the waiter at %s" % (self.name, self.restaurant)
p1 = Place(name='Demon Dogs', address='944 W. Fullerton')
p1.save()
p2 = Place(name='Ace Hardware', address='1013 N. Ashland')
p2.save()
r = Restaurant(place=p1, serves_hot_dogs=True, serves_pizza=False)
r.save()
r.place
<Place: Demon Dogs the place>
p1.restaurant
<Restaurant: Demon Dogs the restaurant>
hasattr(p2, 'restaurant')
False
r.place = p2
r.save()
p2.restaurant
<Restaurant: Ace Hardware the restaurant>
r.place
<Place: Ace Hardware the place>
p1.restaurant = r
p1.restaurant
<Restaurant: Demon Dogs the restaurant>
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.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>]>
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>
w = r.waiter_set.create(name='Joe')
w
<Waiter: Joe the waiter at Demon Dogs the restaurant>
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>]>
