referential integrity for insert

i got two tables with the following structures:
create table unit (
unit_id int4 not null primary key,
unit_name varchar(10) not null)

create table plant_unit (
pu_id int4 not null primary key,
pu_code varchar(15),
unit_id int4 not null references unit (unit_id)
match all on delete restrict on update cascade)

that means the two tables are connected to each other
by the unit_id field. i cannot delete records from the
unit table if there is a relation in the plant_unit
table, and when i update the unit_id field in the unit
table the system will update all matching records in
the plant_unit table as well.

my question is how can i avoid to insert record into
the plant_unit table, when the specified unit_id is
not available in the unit table.

do i have to use trigger, or i miss something in the
constraint, or whatelse?

thanks

Ivan

__________________________________________________
Do You Yahoo!?
Try FREE Yahoo! Mail - the world's greatest free email!
http://mail.yahoo.com/