1.一对多的问题

1.1 一个用户和他的访问记录就是一个一对多的关系，一对多只需要在多的那个表写语句即可

class Record(models.Model):
    """浏览记录"""
    recordid = models.BigAutoField(primary_key=True)
    # 一对多 ，在多的一方写ForeignKey语句，to表示对应的表，on_delete是否关联删除，db_column表示数据库中对应的列
    user = models.ForeignKey(to=User, on_delete=models.PROTECT, db_column='userid')
    recorddate = models.DateTimeField(auto_now=True)

    class Meta:
        managed = False
        db_table = 'tb_record'

1.2 一对多中的一个特例（自参照）。列如省份和市和区都属于地区。区和市，市和省都属于多对一的关系。

class District(models.Model):
    """地区"""
    distid = models.IntegerField(primary_key=True)
    parent = models.ForeignKey(to='self', on_delete=models.PROTECT, db_column='pid', blank=True, null=True)
    name = models.CharField(max_length=255)
    ishot = models.BooleanField(default=False)
    intro = models.CharField(max_length=255, blank=True, null=True)

    def __str__(self):
        return self.name

    class Meta:
        managed = False
        db_table = 'tb_district'

2.对多对的问题

比如现在我们有一个用户表和一个角色表，用户和角色是多对多的关系，那么我们就需要一个中间表用户角色表来将她们关联起来：

class User(models.Model):
    """用户表"""
    userid = models.AutoField(primary_key=True)
    username = models.CharField(unique=True, max_length=20)
    password = models.CharField(max_length=32)
    # ManyToManyField  to表示关联哪张表，through则对应中间表
    roles = models.ManyToManyField(to='Role', through='UserRole')

    class Meta:
        managed = False
        db_table = 'tb_user'


class Role(models.Model):
    """角色表"""
    roleid = models.AutoField(primary_key=True)
    rolename = models.CharField(max_length=255)

    def __str__(self):
        return self.rolename

    class Meta:
        managed = False
        db_table = 'tb_role'


class UserRole(models.Model):
    """用户角色中间实体"""
    urid = models.AutoField(primary_key=True)
    user = models.ForeignKey(to=User, on_delete=models.PROTECT, db_column='userid')
    role = models.ForeignKey(to=Role, on_delete=models.PROTECT, db_column='roleid')

    class Meta:
        managed = False
        db_table = 'tb_user_role'
        # 唯一约束，user可以有多个，role也可以有多个，但一个用户对应的一个user只能有一个
        unique_together = (('user', 'role'), )

3. 一对一问题

class UserToken(models.Model):
    """用户身份令牌"""
    tokenid = models.AutoField(primary_key=True)
    token = models.CharField(max_length=32)
    user = models.OneToOneField(to=User, on_delete=models.PROTECT, db_column='userid')

    class Meta:
        managed = False
        db_table = 'tb_user_token'