Forum OpenACS Development: cr_child_rels sql

I am not quite sure about it, but shouldn' parent_id and child_id have a foreign key restraint???

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create table cr_child_rels (
  rel_id            integer
                    constraint cr_child_rels_rel_pk
                    primary key
                    constraint cr_child_rels_rel_fk
                    references acs_objects,
  parent_id          integer
                    constraint cr_child_rels_parent_nil
                    not null,
  child_id          integer
                    constraint cr_child_rels_child_nil
                    not null,
  relation_tag      varchar(100),
  order_n            integer
);