在關(guān)系數(shù)據(jù)庫(kù)中設(shè)置表與表之間的關(guān)系
Many-to-one的關(guān)系
定義一個(gè)多對(duì)一的關(guān)系蜜另，用django.db.models.ForeignKey燎斩。像用其他字段類(lèi)型意向方便滚躯。ForeignKey有一個(gè)必須的參數(shù)：此類(lèi)關(guān)聯(lián)的model
from diango.db import models
class Manufacturer(models.Model):
pass
class Car(model.Model):
manufacturer = models.ForeignKey(Manufacturer,on_delete=models.CASCADE)
>>>
一個(gè)與自身有多對(duì)一關(guān)系的對(duì)象
models.ForeignKey('self', on_delete=models.CASCADE)
可以通過(guò)Abstract base class與多個(gè)子類(lèi)建立多對(duì)一關(guān)系
products/models.py
from django.db import models
class AbstractCar(models.Model):
manufacturer = models.ForeignKey('Maufacturer',on_delete=models.CASCADE)
class Meta:
abstract = True
production/models.py
from django.db import models
from products.models import AbstractCar
class Manufacturer(models.Model):
pass
class Car(AbstractCar):
pass
class BENSCar(AbstractCar):
pass
>>>Car.manufacturer will point to `production.Manufacturer` here.
ForeignKey還接受一些其他的參數(shù)，用于定義關(guān)系伤柄。
- ForeignKey.on_delete
當(dāng)一個(gè)被引用的對(duì)象被刪除的時(shí)候并扇，Django會(huì)通過(guò)on_delete的參數(shù)模仿SQL的相關(guān)的操作
.CASCADE 當(dāng)被引用的對(duì)象被刪除的時(shí)候去团，此引用對(duì)象也要一并執(zhí)行刪除的操作
.PROTECT 禁止刪除并產(chǎn)生一個(gè)異常(ProtectedError)
.SET_NULL 把外鍵設(shè)置為null，規(guī)劃時(shí)此字段要設(shè)置可接受null
.SET_DEFAULT 把外鍵設(shè)置為默認(rèn)值穷蛹，規(guī)劃是此字段要設(shè)置一個(gè)默認(rèn)值
設(shè)置其他值 采用一個(gè)方法的形式 callable
from django.conf import settings
from django.contrib.auth import get_user_model from django.db import models
def get_sentinel_user():
return get_user_model().objects.get_or_create(username='deleted')[0]
class MyModel(models.Model): user = models.ForeignKey(
settings.AUTH_USER_MODEL,
on_delete=models.SET(get_sentinel_user), )
.DO_NOTHING 什么也不做
- ForeignKey.limit_choices_to
限制字段可供選擇的范圍土陪。參數(shù)是字典的形式或者一個(gè)Q對(duì)象
staff_member = models.ForeignKey(User,on_delete=models.CASCADE,limit_choices_to={'is_staff':True},)
限制了staff_member只有User的is_staff的字段可以作為外鍵關(guān)聯(lián)的記錄
- ForeignKey.related_name
關(guān)聯(lián)對(duì)象反向引用描述符
當(dāng)一張表的多個(gè)字段指向同一張表時(shí)，會(huì)出錯(cuò)俩莽。系統(tǒng)無(wú)法知道旺坠，通過(guò)另外一張表乔遮，訪問(wèn)XXX_set屬性訪問(wèn)到的是哪個(gè)屬性扮超。這時(shí)，我們就需要為每個(gè)字段定義一個(gè)related_name屬性蹋肮，另外一張表訪問(wèn)這個(gè)表時(shí)出刷，就會(huì)根據(jù)related_name的值來(lái)得到各個(gè)屬性了。
Many-to-many
定義一個(gè)Many-to-many關(guān)系使用ManyToManyField坯辩。ManyToManyField有一個(gè)比傳參數(shù)馁龟。
from django.db import models
class Topping(models):
pass
class Pizza(models.Model):
topping = models.ManyToManyField(Topping)
However, sometimes you may need to associate data with the relationship between two models.
用兩個(gè)模型的關(guān)系模型將此二者的數(shù)據(jù)聯(lián)系起來(lái)
例如：音樂(lè)家和音樂(lè)樂(lè)團(tuán)之間的關(guān)系。他們是多對(duì)多的關(guān)系漆魔，但是關(guān)于這個(gè)會(huì)員資格的一些信息需要一個(gè)額外的數(shù)據(jù)模型來(lái)解釋坷檩。Django允許指定一個(gè)model來(lái)管理這種關(guān)系，設(shè)置一些字段在這個(gè)中間模型里改抡。
from django.db import models
class Person(models.Model):
name = models.CharField(max_length=128)
def __str__(self):
return self.name
class Group(models.Model):
name = models.CharField(max_length=128)
members = models.ManyToManyField(Person,through='Membership')
def __str__(self):
return self.name
class Membership(models.Model):
person = models.ForeignKey(Person,on_delete=models.CASCADE)
group = models.ForeignKey(Group,on_detele=models.CASCADE)
date_joined = models.DateField()
invite_reason = models.CharField(max_length=64)
>>>其中Membership就是中間model矢炼。中間model中ForeignKey把兩個(gè)有關(guān)聯(lián)的model聯(lián)系起來(lái)
>>> ringo = Person.objects.create(name="Ringo Starr") >>> paul = Person.objects.create(name="Paul McCartney") >>> beatles = Group.objects.create(name="The Beatles") >>> m1 = Membership(person=ringo, group=beatles,
... date_joined=date(1962, 8, 16),
... invite_reason="Needed a new drummer.")
>>> m1.save()
>>> beatles.members.all()
<QuerySet [<Person: Ringo Starr>]>
>>> ringo.group_set.all()
<QuerySet [<Group: The Beatles>]>
>>> m2 = Membership.objects.create(person=paul, group=beatles,
... date_joined=date(1960, 8, 1),
... invite_reason="Wanted to form a band.")
>>> beatles.members.all()
<QuerySet [<Person: Ringo Starr>, <Person: Paul McCartney>]>
在有中間model的多對(duì)多的關(guān)系中，往常的add(),create(),set()不起作用
>>> # The following statements will not work
>>> beatles.members.add(john)
>>> beatles.members.create(name="George Harrison")
>>> beatles.members.set([john, paul, ringo, george])
只能通過(guò)創(chuàng)建中間Model的實(shí)例的方式添加新的關(guān)系
在查詢(xún)的時(shí)候阿纤，與普通的Many-to-Many的關(guān)系一樣
# Find all the members of the Beatles that joined after 1 Jan 1961
>>> Person.objects.filter(
... group__name='The Beatles',
... membership__date_joined__gt=date(1961,1,1))
<QuerySet [<Person: Ringo Starr]>
>>> ringos_membership = Membership.objects.get(group=beatles, person=ringo)
>>> ringos_membership.date_joined
datetime.date(1962, 8, 16)
>>> ringos_membership.invite_reason
'Needed a new drummer.'
One-to-one relationships
用OneToOneField來(lái)定義一個(gè)一對(duì)一的關(guān)系句灌，需要傳遞一個(gè)把綁定的model
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(model.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)
p1 = Place(name='Demon Dogs',address='994 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 #由于p1與r關(guān)聯(lián)起來(lái)了，可以通過(guò)p1.resaurant的方式找到p1關(guān)聯(lián)的resaurant欠拾。
<Restaurant: Demon Dogs the restaurant>
添加一個(gè)Waiter
w = r.waiter_set.create(name='Joe')
