ORA-04084: cannot change NEW values for this trigger type

Question

I'm trying to turn pl/sql trigger that calculates the total of some cells in the table when the tale is changed. This is the code:

  ALTER session SET nls_date_format='dd/mm/yyyy';

  CREATE OR REPLACE TRIGGER TOTAL
  AFTER UPDATE OR INSERT ON ORDER_ITEMS
  FOR EACH ROW
    DECLARE
temp  NUMBER;
today DATE;
  BEGIN
         temp:=(:NEW.item_price-:NEW.discount_amount)*:NEW.quantity;
         today := CURRENT_DATE;
        :NEW.TOTAL := temp;
          dbms_output.put_line('Updated on:' ||today || ' item number: ' ||:NEW.item_id|| 'order number:' ||:NEW.order_id|| 'total: ' ||:NEW.total);
  END;
  /
  show errors

  insert into order_items (ITEM_ID, ORDER_ID, PRODUCT_ID, ITEM_PRICE, discount_amount, QUANTITY)
  VALUES (13, 7, 3, 553, 209, 2);

And I get this error:

4084. 00000 - "cannot change NEW values for this trigger type" *Cause: New trigger variables can only be changed in before row insert or update triggers. *Action: Change the trigger type or remove the variable reference. No Errors. 1 rows inserted Updated on:06/01/2016 item number: 13order number:7total:

I understand that the problem is updating a table during the trigger execution caused by an update to the same table.

Answer 1

As requested in comments I'm making my comment as an answer.

Your problem is because you are trying to change a value AFTER the value was persisted, try changing your trigger to BEFORE as:

CREATE OR REPLACE TRIGGER TOTAL
  BEFORE UPDATE OR INSERT ON ORDER_ITEMS
  FOR EACH ROW
DECLARE
  temp  NUMBER;
  today DATE;
BEGIN
    temp:=(:NEW.item_price-:NEW.discount_amount)*:NEW.quantity;
    today := CURRENT_DATE;
    :NEW.TOTAL := temp;
    dbms_output.put_line('Updated on:' || today || ' item number: '
                           || :NEW.item_id || 'order number:' || :NEW.order_id 
                           || 'total: ' ||:NEW.total);
END;
/