Postgres trigger function

Question

I need help in Postgres triggers.

I have table with 2 columns:

sold boolean;
id_shop int;

It stores if item is sold, or at which shop its located at.

I need a trigger, if I change the "sold" to true, then it also changes the id_shop to NULL (It can't be in shop if sold...)

I tried different ways, but it doesn't work or gives an error on update cmd...

create or replace function pardota_masina_veikals() RETURNS trigger AS $pardota_masina$
begin
  IF NEW.sold=true THEN
    update masinas
      SET id_shop=null WHERE id=NEW.id;
  END IF;
  RETURN NEW;
END;
$pardota_masina$ LANGUAGE plpgsql;


CREATE TRIGGER pardota_masina_nevar_but_veikala 
    AFTER INSERT OR UPDATE ON masinas FOR EACH ROW EXECUTE PROCEDURE pardota_masina_veikals();

Answer 1

First of all you need a before trigger if you want to change a value of the row being updated (or inserted)

Secondly you don't need to "update" the table, just assign the new value to the NEW row:

create or replace function pardota_masina_veikals() 
RETURNS trigger 
AS 
$pardota_masina$
begin
  IF NEW.sold=true THEN
    NEW.id_shop = NULL;
 END IF;
RETURN NEW;
END;
$pardota_masina$ 
LANGUAGE plpgsql;

CREATE TRIGGER pardota_masina_nevar_but_veikala 
   BEFORE INSERT OR UPDATE ON masinas 
   FOR EACH ROW EXECUTE PROCEDURE pardota_masina_veikals();