pythondjangodjango-querysetdjango-ormdjango-annotate

How to subtract two annotated columns on Django QuerySets?

I need to be able to sort on the aggregate of two annotated columns

So I'd like to do something like this:

c = c.annotate(metric=Sum('results__metric'))
c = c.annotate(metric_prior=Sum('results__metric_prior'))
c = c.annotate(variance=F('metric')-F('metric_prior')) #doesn't work, for demonstrative purposes only

and then:

c = c.order_by('variance')

Does anyone know how to accomplish something like the above?

Solution

  • Actually,

    c = c.annotate(variance=F('metric')-F('metric_prior'))

    works as you would like it to starting with Django 1.8.

    Moreover, you can also order by an expression, which means you can just use:

    c = c.order_by(F('metric') - F('metric_prior'))

    or even just:

    c = c.order_by(Sum('results__metric') - Sum('results__metric_prior'))