Django_数据模型中on_delete使用

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

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.

二、on_delete外键删除方式

• CASCADE：级联删除。当Manufacturer对象删除时，它对应的Car对象也会删除。
• PROTECT：保护模式，采用该选项，删除时会抛出ProtectedError错误。
• 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
• 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。
• SET()：自定义一个值，该值当然只能是对应的实体了