constraint function, newbie

hi
i am learning postgres and made a library model

i dont want a user to be able to borrow 2 items (copies)
of the same book so made a function to check it (below)

the function works fine but now i would like to add a constraint,
unfortunatelly i fail

here's my try:

create function bookborrow_check_insert
(bookborrow.userid%TYPE, bookborrow.bookitem_id%TYPE)
returns bool as
'select NOT $1 in (select userid from bookborrow where userid = $1 and
bookitem_id = $2 and returned is null)'
 language sql;

=> alter table bookborrow add check(bookborrow_check_insert (userid, bookitem_id));
ERROR:  AlterTableAddConstraint: rejected due to CHECK constraint <unnamed>

=> alter table bookborrow add  constraint check_insert check(bookborrow_check_insert (userid, bookitem_id));
ERROR:  AlterTableAddConstraint: rejected due to CHECK constraint check_insert

--
Michal Adamczak
pokryfka @ druid.if.uj.edu.pl
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