Constraints...

Does anyone know how I can make a constraint on a key to enforce a 1 
to n relationship where n>0?

I've invented an example to show the sort of constraint I need:
CREATE TABLE permissions ( id int4, userid int4, perm int4, primary key (id,userid)
);
CREATE TABLE objects ( id int4, perm int4 NOT NULL, data text
);

INSERT INTO permissions (id,userid,perm) VALUES (1,1001,7);
INSERT INTO permissions (id,userid,perm) VALUES (1,1002,6);
INSERT INTO permissions (id,userid,perm) VALUES (1,1003,4);
INSERT INTO permissions (id,userid,perm) VALUES (2,1001,0);

So I want to allow something like:
INSERT INTO objects (id,perm,data) VALUES (1,1,'everyone can read');
INSERT INTO objects (id,perm,data) VALUES (2,1,'everyone can read');
INSERT INTO objects (id,perm,data) VALUES (3,2,'nobody can read');

But disallow an insert like:
INSERT INTO objects (id,perm,data) VALUES (9999,1,'bad perm example');

Is this possible? 

-Michael

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