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 modelsclass Manufacturer(models.Model): # ... passclass 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') create_at = models.CharField(max_length=20, verbose_name='創建時間') 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)

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

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

因此,整理一下on_delete參數的各個值的含義:

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行對象的返回值,設置：models.SET(可執行對象)

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)

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

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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