Django F() division - How to avoid rounding off

Question

I have this code:

q = MyModel.objects.order_by(\'-value1\').annotate(
            res=ExpressionWrapper(
                (F(\'value1\') / F(\'value2\')), 
               


        

Answer 1

In v1.10, Django introduced a Cast function which makes this (almost) a breeze – the qualification being that you either need two casts or need to wrap the whole thing in an ExpressionWrapper to avoid a FieldError: Expression contains mixed types. You must set output_field.

q = MyModel.objects.annotate(
            res=ExpressionWrapper(
                (Cast('value1', FloatField()) / F('value2')), 
                output_field=FloatField()),
            )

This is functionally equivalent to multiplying the enumerator by 1.0, but an explicit cast makes your intention clearer than a multiplication by a number.