ForeignKey中on_delete

"on_delete=models.SET_NULL"是Django模型中字段的一个选项，用于指定在与该字段关联的对象被删除时如何处理它。

具体来说，"on_delete"选项有多种可选值，包括：

CASCADE：级联删除，即删除与该字段相关的所有对象。

PROTECT：保护模式，即防止删除关联对象，抛出ProtectedError异常。

SET_NULL：将该字段设为NULL

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