ex.sql

 -- Remove the old score from that costumer in that product
 DROP TRIGGER IF EXISTS new_score_before ON stores;
 DROP FUNCTION IF EXISTS new_score_before();
 CREATE OR REPLACE FUNCTION new_score_before() RETURNS trigger as $$
    BEGIN
            DELETE FROM products_scores
            WHERE NEW.user_id = products_scores.user_id AND
                  NEW.product_id = products_scores.product_id;  
            RETURN NEW;
    END;
$$ LANGUAGE plpgsql;

CREATE TRIGGER new_score_before BEFORE INSERT ON products_scores
    FOR EACH ROW EXECUTE PROCEDURE new_score_before();