on_delete屬性針對外來鍵ForeignKey

一、django3.0官方文件介紹：

Many-to-one relationships多對一關係

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.

常見的使用方式(設定為null)

class ApiList(models.Model):
 desc = models.CharField(max_length=255,verbose_name="介面描述")
 keyword = models.CharField(max_length=100,verbose_name="請求關鍵字")
 response = models.TextField(verbose_name="響應結果")
 api = models.ForeignKey(Api,blank=True,null=True,on_delete=models.SET_NULL,verbose_name="所屬介面")
 status = models.IntegerField(default=1,verbose_name="狀態")
 create_at = models.CharField(max_length=20,verbose_name="建立時間")
 update_at = models.CharField(max_length=20,verbose_name="更新時間")

一對多(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,from_fields=['self'],to_fields=[to_field],**kwargs)

一對一(OneToOneField)

class OneToOneField(ForeignKey):
  def __init__(self,**kwargs):
    kwargs['unique'] = True
    super().__init__(to,to_field=to_field,**kwargs)

從上面外來鍵(ForeignKey)和一對一(OneToOneField)的引數中可以看出,都有on_delete引數,而 django 升級到2.0之後,表與表之間關聯的時候,必須要寫on_delete引數,否則會報異常:

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

因此,整理一下on_delete引數的各個值的含義:

on_delete=None,# 刪除關聯表中的資料時,當前表與其關聯的field的行為
on_delete=models.CASCADE,# 刪除關聯資料,與之關聯也刪除
on_delete=models.DO_NOTHING,什麼也不做
on_delete=models.PROTECT,引發錯誤ProtectedError
# models.ForeignKey('關聯表',null=True)
on_delete=models.SET_NULL,與之關聯的值設定為null（前提FK欄位需要設定為可空,一對一同理）
# models.ForeignKey('關聯表',on_delete=models.SET_DEFAULT,default='預設值')
on_delete=models.SET_DEFAULT,與之關聯的值設定為預設值（前提FK欄位需要設定預設值,一對一同理）
on_delete=models.SET,a. 與之關聯的值設定為指定值,設定：models.SET(值)
 b. 與之關聯的值設定為可執行物件的返回值,設定：models.SET(可執行物件)

多對多(ManyToManyField)

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

因為多對多(ManyToManyField)沒有 on_delete 引數,所以略過不提.

二、on_delete外來鍵刪除方式

1. CASCADE：級聯刪除。當Manufacturer物件刪除時，它對應的Car物件也會刪除。
2. PROTECT：保護模式，採用該選項，刪除時會丟擲ProtectedError錯誤。
3. SET_NULL：置空模式，刪除的時候，外來鍵欄位被設定為空，前提就是blank=True,定義該欄位的時候，允許為空。當Manufacturer物件刪除時，它對應的Car物件的manufacturer欄位會置空，前提是null=True
4. SET_DEFAULT：置預設值，刪除的時候，外來鍵欄位設定為預設值，所以定義外來鍵的時候注意加上一個預設值。
5. SET()：自定義一個值，該值當然只能是對應的實體了

django3.0關於models官方文件地址：
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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