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):
    A = models.IntegerField(..)
    B = models.IntegerField(..)
    C = models.ForeignKey(..)

I want to have a D and an E field that are calculated by the following formulas:

1. D = A - B
2. E = A - X (where X is a field of the relevant record of model C)

Implementing this would be trivial if I didn't need to apply sorting; I would just add properties to the model class. However, I need ordering by these fields.

A solution is to fetch all records into memory and do the sorting there, which I conceive a last resort (it will break things regarding pagination).

Is there a way to achieve what I'm trying? Any guidance is appreciated.

EDIT: Denormalization is a no-go. The value of field X changes very frequently and a lot of Foo records are related to one record of model C. An update of X will require thousands of updates of E.

Answer 1

I would take a look at the extra method on Queryset and specify the order_by parameter.

Answer 2

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.

Answer 3

If you would not mind some logic duplicaton, then the following will work:

Foo.objects.extra(select={ 'd_field' : 'A - B' }).extra(order_by=['d_field'])