Django數據模型中on_delete使用詳解

更新時間：2020年11月30日 15:14:14 作者：湯圓兒2019

這篇文章主要介紹了Django數據模型中on_delete使用詳解，文中通過示例代碼介紹的非常詳細，對大家的學習或者工作具有一定的參考學習價值，需要的朋友們下面隨著小編來一起學習學習吧

on_delete屬性針對外鍵ForeignKey

一、django3.0官方文檔介紹：

Many-to-one relationships多對一關系

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.

常見的使用方式(設置為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="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="所屬接口")
 status = models.IntegerField(default=1, verbose_name="狀態(tài)")
 create_at = models.CharField(max_length=20, verbose_name="創(chuàng)建時間")
 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, on_delete, from_fields=['self'], to_fields=[to_field], **kwargs)

一對一(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)

on_delete=None,        # 刪除關聯表中的數據時,當前表與其關聯的field的行為
on_delete=models.CASCADE,   # 刪除關聯數據,與之關聯也刪除
on_delete=models.DO_NOTHING, # 刪除關聯數據,什么也不做
on_delete=models.PROTECT,   # 刪除關聯數據,引發(fā)錯誤ProtectedError
# models.ForeignKey('關聯表', on_delete=models.SET_NULL, blank=True, null=True)
on_delete=models.SET_NULL,  # 刪除關聯數據,與之關聯的值設置為null（前提FK字段需要設置為可空,一對一同理）
# models.ForeignKey('關聯表', on_delete=models.SET_DEFAULT, default='默認值')
on_delete=models.SET_DEFAULT, # 刪除關聯數據,與之關聯的值設置為默認值（前提FK字段需要設置默認值,一對一同理）
on_delete=models.SET,     # 刪除關聯數據,
 a. 與之關聯的值設置為指定值,設置：models.SET(值)
 b. 與之關聯的值設置為可執(zhí)行對象的返回值,設置：models.SET(可執(zhí)行對象)

多對多(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)

二、on_delete外鍵刪除方式

CASCADE：級聯刪除。當Manufacturer對象刪除時，它對應的Car對象也會刪除。
PROTECT：保護模式，采用該選項，刪除時會拋出ProtectedError錯誤。
SET_NULL：置空模式，刪除的時候，外鍵字段被設置為空，前提就是blank=True, null=True,定義該字段的時候，允許為空。當Manufacturer對象刪除時，它對應的Car對象的manufacturer字段會置空，前提是null=True
SET_DEFAULT：置默認值，刪除的時候，外鍵字段設置為默認值，所以定義外鍵的時候注意加上一個默認值。
SET()：自定義一個值，該值當然只能是對應的實體了

django3.0關于models官方文檔地址：
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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