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.