确保字段在Django中始终为null [英] Making sure a field is always null in Django
问题描述
我有一个父类，将通过3个不同的模型进行扩展。可以说，父类有一个名为 foo 的字段，并且对于子类之一，该字段必须始终为null。我该如何确保？现在，我正在使用 null = True 和 editable = False 约束。但是，如果在创建对象的过程中暴露了该字段，则可以从外壳程序或API中规避这些问题。
I have a parent class which is being extended by 3 different models. The parent class has a field called foo lets say, and this field needs to be always null for one of the sub-classes. How can I ensure this? Right now, I am using null=True, and editable=False constraints. However, these can be circumvented from the shell or an API if the field is exposed during object creation.
class ThirdSubclass(ParentClass):
# Over-ridden field from abstract parent class
foo = models.PositiveSmallIntegerField(blank=True, null=True, editable=False)
我还使用了PositiveSmallIntegerField，因为我想为该字段分配尽可能少的空间。无论如何，我该怎么做？覆盖保存方法是唯一的选择吗？理想情况下，我会喜欢字段定义中的某些内容。谢谢！
I am also using the PositiveSmallIntegerField since I want to allocate as little space as possible for this field. Anyway, how do I do this? Is over-riding the save method the only option? Ideally, I would love something within the field definition. Thanks!
推荐答案
正确的方法取决于您期望模型的使用方式。以下是四种可能的方法，以提高健壮性的顺序：
The right way to do this depends on how you expect your models to be used. Here are four possible approaches, in order of increasing robustness:
-
如果所有写入都将通过默认模型形式进行：
If all writes will come through default model forms:
您可以设置默认值：
class ThirdSubclass(ParentClass):
foo = models.PositiveSmallIntegerField(blank=True, null=True, default=None, editable=False)
如果所有写入都将通过使用模型验证来完成（即调用 full_clean（） 在 save（））：
您可以使用验证器：
def validate_none(value):
if value is not None:
raise ValidationError("...")
class ThirdSubclass(ParentClass):
foo = models.PositiveSmallIntegerField(blank=True, null=True, default=None, editable=False,
validators=[validate_none])
如果所有写入都会通过该模型：
If all writes will come through the model:
您可以覆盖 save（）。
class ThirdSubclass(ParentClass):
def save(self, *args, **kwargs):
if self.foo is not None:
raise ValueError("...")
super().save(*args, **kwargs)
如果写入可以来自任何地方（例如，使用 update（） 或原始SQL）：
If writes can come from anywhere (for example, using update() or raw SQL):
您需要数据库级约束。 Django没有为此提供模型级别的支持，但是您可以通过编写自定义迁移并使用 RunSQL 创建约束来创建一个。
You need a database-level constraint. Django has no model-level support for this, but you could create one by writing a custom migration and using RunSQL to create the constraint.
其中我认为2是标准的，如果您不需要3和4的保护，则是最优雅的。
Of these I would consider 2 to be standard, and the most elegant if you don't need the protections of 3 and 4.
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