django - ordering queryset by a calculated field

Question

I want to have a model with calculated fields that I can apply sorting on. For example, let\'s say that I have the following model:

class Foo(models.Model):
         


        

Answer 1

I haven't presently got a Django install running, but I think what you're asking is how to do a custom save, such that D and E are automatically generated. I don't know what your ForeignKey's return on unicode is, so I'm assuming it's not a string and assigning "valueName" as token vlaue for the integer you want to usage.

Anyway, it should go a bit like this:

class Foo(models.Model):
    A = models.IntegerField(..)
    B = models.IntegerField(..)
    C = models.ForeignKey(..)
    D = models.IntegerField(..)
    E = models.IntegerField(..)
    def save(self):
        self.D = self.A - self.B
        self.E = self.A - self.C.valueName
        super(Foo, self).save()

Anything prior to the last line of that (super()) will be PRE save, anything after is POST. That's really the most important point there.