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  • django2.0 关联表的必填on_delete参数的含义

    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.

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  • 原文地址:https://www.cnblogs.com/zl-py/p/10585090.html
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