在表单或者模型中不属于特定字段的`ValidationError` 被归类为`NON_FIELD_ERRORS`。This constant is used as a key in dictionaries that otherwise map fields to their respective list of errors.
使用[`django.db.models.PROTECT`](models/fields.html#django.db.models.PROTECT"django.db.models.PROTECT")时,抛出异常来阻止所引用对象的删除。[`models.`](#django.db.models.ProtectedError"django.db.models.ProtectedError")ProtectedError is a subclass of [`IntegrityError`](#django.db.IntegrityError"django.db.IntegrityError").