调用models.ForeignKey时报“TypeError: init() missing 1 required positional argument: 'on_delete'”

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在执行python manage.py makeigrations app_name时遇到'TypeError: __init__() missing 1 required positional argument: 'on_delete''错误。问题源于Django的ForeignKey字段必须包含'on_delete'参数。解决方案是在定义外键时指定'on_delete'行为，如models.ForeignKey(OtherModel, on_delete=models.CASCADE)。" 138719416,16848851,"C++编程技巧：explicit, 静态成员与常量成员，vector与array对比

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在运行 python manage.py makeigrations app_name时，若调用models.ForeignKey时参数里除了对应的model没有其他参数时，会报“TypeError: __init__() missing 1 required positional argument: 'on_delete'”的错误。

究其原因，发现在virt_env\Lib\site-packages\django\db\models\fields\related.py的class ForeignKey(ForeignObject)中，有如下check.

    def _check_on_delete(self):
        on_delete = getattr(self.remote_field, 'on_delete', None)
        if on_delete == SET_NULL and not self.null:
            return [
                checks.Error(
                    'Field specifies on_delete=SET_NULL, but cannot be null.',
                    hint='Set null=True argument on the field, or change the on_delete rule.',
                    obj=self,
                    id='fields.E320',
                )
            ]
        elif on_delete == SET_DEFAULT and not self.has_default():
            return [
                checks.Error(
                    'Field specifies on_delete=SET_DEFAULT, but has no default value.',
                    hint='Set a default value, or change the on_delete rule.',
                    obj=self,
                    id='fields.E321',
                )
            ]
        else:
            return []

解决方法之一：在参数里加上如下参数“,on_delete=CASCADE,”或者“,on_delete=DO_NOTHING,"。