I've been trying to use this feature but I'm not being able to make it work. I'm using this syntax:
relation_aggregate(relation := 'tubo_et_face_et_inicial', aggregate := 'sum', expression := "lenght_linear")
to try to sum up all the values in the "lenght_linear field of the child table, to a virtual field in the parent table. Is this syntax correct, or am I doing something wrong?