Annotate a sum of two fields multiplied

Maybe you don’t need this answer now, but if you read the documentation about Sum expression , you need to declare the output_field, like this:

Customer.objects.filter(something)
                .annotate(total_spent=Sum(
                    F('order__lineitem__quantity') * 
                    F('order__lineitem__price'),   
                    output_field=models.FloatField()
                ))