Error in PostgreSQL trigger - postgresql

Question

i'm coding this trigger in postgreSQL
CREATE OR REPLACE FUNCTION fn_trg_viabilidad_fila()
RETURNS trigger AS
$BODY$
BEGIN
PERFORM S.*
FROM MontoMinimo M, SolicitudPresupuesto S, Cantidad C, Producto P
WHERE P.idProducto=C.idProducto
and C.idPresupuesto=S.idPresupuesto
and M.idMonto=S.idMonto;
IF (C.cantidad < P.canMinExp OR P.exportable = FALSE)
THEN
UPDATE SolicitudPresupuesto
SET viable = FALSE
WHERE idPresupuesto = OLD.idPresupuesto;
RETURN NEW;
END IF;
END
$BODY$
LANGUAGE plpgsql
CREATE TRIGGER trg_viabilidad_fila BEFORE INSERT
OR UPDATE ON SolicitudPresupuesto
FOR EACH ROW EXECUTE PROCEDURE
fn_trg_viabilidad_fila() ;
I can't solve this error..
An error has occurred: ERROR: missing FROM-clause entry for table "c"
LINE 1: SELECT C.cantidad < P.canminexp OR P.exportable = FALSE ^
QUERY: SELECT C.cantidad < P.canminexp OR P.exportable = FALSE
CONTEXT: PL/pgSQL function fn_trg_viabilidad_fila() line 9 at IF
I will be very grateful to any help. Sorry for my bad english

Answer 1

You can't access the columns of a query outside of the query (or the block where you use the query). You need to store the result of the select somewhere. Additionally you shouldn't run an UPDATE on the triggered table, you need to assign the value to the NEW record.
CREATE OR REPLACE FUNCTION fn_trg_viabilidad_fila()
RETURNS trigger AS
$BODY$
DECLARE
l_result boolean;
BEGIN
SELECT (c.cantidad < p.canMinExp OR p.exportable = FALSE)
INTO l_result
FROM MontoMinimo M
JOIN SolicitudPresupuesto s ON m.idMonto = s.idMonto
JOIN Cantidad c ON c.idPresupuesto = s.idPresupuesto
JOIN Producto p ON p.idProducto = c.idProducto;
IF l_result THEN
new.viable := false;
END IF;
RETURN NEW;
END
$BODY$
LANGUAGE plpgsql;
It would be possible to "inline" the query into the IF statement but this way it resembles the structure of your current code better. Also note that I replaced the old, outdated implicit joins by an explicit and more robust JOIN operator.
The assigment new.viable assumes that idpresupuesto is the PK in the table solicitudpresupuesto (because you used that in the WHERE clause of the UPDATE statement)