derp_r1_sum_select = """
    select sum(`derp_related_1`.`amount`)
    from `derp_related_1` 
    where `derp_related_1`.`fk` = `derp`.`pk`
    """
derp_r2_sum_select = """
    select sum(`derp_related_2`.`amount`)
    from `derp_related_2`
    where `derp_related_2`.`fk` = `derp`.`pk`"
    """

def queryset(self, request):
    qs = super(DerpAdmin, self).queryset(request)
Django aggregation across multiple tables in ModelAdmin queryset