Relationship

• 主外关系: 关联操作
  • models.CASCADE 级联删除 和 models.SET_NULL 设置为 NULL
• 一对多关系: ForiegnKey 写在一对多关系中，多的那个模型中

一对多

使用 django.db.models.ForiegnKey

from django.db import models
class VolksWagenwerk(models.Model):                  
    name = models.CharField(max_length=20)
class Benz(models.Model):
    volkswagenwerk = models.ForiegnKey(           # 将外键写在多
        VolksWagenwerk, 
        on_delete=models.CASCADE
    )   
    name = models.CharField(max_length=20)

• 外键名是对应类名的小写
• on_delete 是必须属性

一对一

使用 django.db.models.OneToOneField

class Place(models.Model):
    name = models.CharField(max_length=50)
    address = models.CharField(max_length=80)
class Fone(models.Model):
    place = models.OneToOneField(
        Place, 
        on_delete=models.CASCADE, 
        primary_key=True                  # 设置该字段为主键
    )
    server_drs = models.BooleanField(default=False)
class Emp(models.Model):
    fone = models.ForeignKey(Fone, on_delete=models.CASCADE)
    name = models.CharField(max_length=20)from django.db import models
class VolksWagenwerk(models.Model):                  
    name = models.CharField(max_length=20)
class Benz(models.Model):
    volkswagenwerk = models.ForiegnKey(VolksWagenwerk, on_delete=models.CASCADE)   
    name = models.CharField(max_length=20)

多对多

使用 django.db.models.ManyToManyField，会生成一个中间表

自关联

class Student(models.Model):
    name = models.CharField(max_length=20)
    friends = models.ManyToManyField('self')

简易多对多

class SchoolClass(models.Model):
    name = models.CharField(max_length=20)
class Teacher(models.Model):
    name = models.CharField(max_length=10)
    school_class = models.ManyToMany(SchoolClass)

自定义中间表

class Person(models.Model):
    name = models.CharField(max_length=20)
class Group(models.Model):
    name = models.CharField(max_length=100)
    member = models.ManyToManyField(
        Person,
        through= 'Membership',                 # 必须是类名的字符串
        through_fields=('group', 'person')
    )
class Membership(models.Model):         # 自定义中间表
    group = models.ForeignKey(Group, on_delete=models.CASCADE)
    person = models.ForeignKey(Person, on_delete=models.CASCADE)
    score = models.IntegerField(default=1)

• 通过 through=’Membership’ 指定 Merbership 作为中间表
• 通过 through_field=(‘group’, ‘person’) 指定中间模型的属性
• 一般需要自定义中间表时，都有额外的字段，譬如 score = models.IntegerField(default=1)