Django数据模型中on_delete使用详解

on_delete属性针对外键ForeignKey

一、django3.0官方文档介绍：

Many-to-one relationships多对一关系

To define a many-to-one relationship, use django.db.models.ForeignKey. You use it just like any other Field type: by including it as a class attribute of your model.

ForeignKey requires a positional argument: the class to which the model is related.

For example, if a Car model has a Manufacturer – that is, a Manufacturer makes multiple cars but each Car only has one Manufacturer – use the following definitions:

from django.db import models

class Manufacturer(models.Model):
  # ...
  pass

class Car(models.Model):
  manufacturer = models.ForeignKey(Manufacturer, on_delete=models.CASCADE)
  # ...

 You can also create recursive relationships (an object with a many-to-one relationship to itself) and relationships to models not yet defined; see the model field reference for details.

It's suggested, but not required, that the name of a ForeignKey field (manufacturer in the example above) be the name of the model, lowercase. You can, of course, call the field whatever you want.

常见的使用方式(设置为null)

class ApiList(models.Model):
 desc = models.CharField(max_length=255, verbose_name="接口描述")
 keyword = models.CharField(max_length=100, verbose_name="请求关键字")
 response = models.TextField(verbose_name="响应结果")
 api = models.ForeignKey(Api, blank=True, null=True, on_delete=models.SET_NULL, verbose_name="所属接口")
 status = models.IntegerField(default=1, verbose_name="状态")
 create_at = models.CharField(max_length=20, verbose_name="创建时间")
 update_at = models.CharField(max_length=20, verbose_name="更新时间")

一对多(ForeignKey)

class ForeignKey(ForeignObject):
  def __init__(self, to, on_delete, related_name=None, related_query_name=None,
         limit_choices_to=None, parent_link=False, to_field=None,
         db_constraint=True, **kwargs):
    super().__init__(to, on_delete, from_fields=['self'], to_fields=[to_field], **kwargs)

一对一(OneToOneField)

class OneToOneField(ForeignKey):
  def __init__(self, to, on_delete, to_field=None, **kwargs):
    kwargs['unique'] = True
    super().__init__(to, on_delete, to_field=to_field, **kwargs)

从上面外键(ForeignKey)和一对一(OneToOneField)的参数中可以看出,都有on_delete参数,而 django 升级到2.0之后,表与表之间关联的时候,必须要写on_delete参数,否则会报异常:

TypeError: __init__() missing 1 required positional argument: 'on_delete'

因此,整理一下on_delete参数的各个值的含义:

on_delete=None,        # 删除关联表中的数据时,当前表与其关联的field的行为
on_delete=models.CASCADE,   # 删除关联数据,与之关联也删除
on_delete=models.DO_NOTHING, # 删除关联数据,什么也不做
on_delete=models.PROTECT,   # 删除关联数据,引发错误ProtectedError
# models.ForeignKey('关联表', on_delete=models.SET_NULL, blank=True, null=True)
on_delete=models.SET_NULL,  # 删除关联数据,与之关联的值设置为null（前提FK字段需要设置为可空,一对一同理）
# models.ForeignKey('关联表', on_delete=models.SET_DEFAULT, default='默认值')
on_delete=models.SET_DEFAULT, # 删除关联数据,与之关联的值设置为默认值（前提FK字段需要设置默认值,一对一同理）
on_delete=models.SET,     # 删除关联数据,
 a. 与之关联的值设置为指定值,设置：models.SET(值)
 b. 与之关联的值设置为可执行对象的返回值,设置：models.SET(可执行对象)

多对多(ManyToManyField)

class ManyToManyField(RelatedField):
  def __init__(self, to, related_name=None, related_query_name=None,
         limit_choices_to=None, symmetrical=None, through=None,
         through_fields=None, db_constraint=True, db_table=None,
         swappable=True, **kwargs):
    super().__init__(**kwargs)

因为多对多(ManyToManyField)没有 on_delete 参数,所以略过不提. 

二、on_delete外键删除方式

1. CASCADE：级联删除。当Manufacturer对象删除时，它对应的Car对象也会删除。
2. PROTECT：保护模式，采用该选项，删除时会抛出ProtectedError错误。
3. SET_NULL：置空模式，删除的时候，外键字段被设置为空，前提就是blank=True, null=True,定义该字段的时候，允许为空。当Manufacturer对象删除时，它对应的Car对象的manufacturer字段会置空，前提是null=True
4. SET_DEFAULT：置默认值，删除的时候，外键字段设置为默认值，所以定义外键的时候注意加上一个默认值。
5. SET()：自定义一个值，该值当然只能是对应的实体了

django3.0关于models官方文档地址：
1.https://docs.djangoproject.com/en/3.0/topics/db/models/
2.https://docs.djangoproject.com/en/3.0/ref/models/fields/#django.db.models.ForeignKey

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