SQL Trigger to pre-insert user_id into message

insert_user_id.sql

CREATE FUNCTION insert_user_id()
    RETURNS trigger AS $BODY$
    BEGIN
    SELECT id INTO NEW.to_id FROM users WHERE email = NEW.to_email;
    RETURN NEW;
    END;
    $BODY$ LANGUAGE plpgsql;

CREATE TRIGGER insert_article BEFORE INSERT OR UPDATE ON messages FOR EACH ROW EXECUTE PROCEDURE insert_user_id();