在Django中为模型定义另一个变量时使用变量

Question

Basically what I want to do is this:

class A(models.Model):
    a_number = models.FloatField()


class B(models.Model):
    a = models.ForeignKey(A)
    b = models.FloatField(default=self.a.a_number)

I understand I can't use self while defining the variables, but is there a workaround for this? What if 'a_number' was a method instead of a variable?

I know i could make a method like this in B:

def b(self):
    return self.a.a_number

But I need to get the value of 'a_number' right as I create the object in B so this wouldn't work.

Answer 1

Implement a clean() method to set the default value for b .

class B(models.Model):
    a = models.ForeignKey(A)
    b = models.FloatField()

    def clean(self):
        if self.b is None:
            self.b = self.a.a_number

The Django documentation linked above suggests clean() as the way to automatically provide a value for a field:

Model.clean()

This method should be used to provide custom model validation, and to modify attributes on your model if desired. For instance, you could use it to automatically provide a value for a field, or to do validation that requires access to more than a single field:

 (example omitted) 

Remember to call clean() before calling save() on your model. If you use the Django admin site, then clean() is automatically called when an object is created or updated on that site.