-- channels.community_id is immutable: a channel can never be re-tenanted.
-- (Conformance: "Migration lint forbids channel re-tenanting except through an
-- explicitly modeled admission path." We have no such path, so: hard block.)
CREATE FUNCTION channels_community_id_immutable() RETURNS TRIGGER AS $$
BEGIN
    IF NEW.community_id IS DISTINCT FROM OLD.community_id THEN
        RAISE EXCEPTION 'channels.community_id is immutable (channel % cannot be re-tenanted)', OLD.id
            USING ERRCODE = 'check_violation';
    END IF;
    RETURN NEW;
END;
$$ LANGUAGE plpgsql;