problem with function trigger

Hi I'm trying to update a table column with a pl/pgsql function and a
trigger.
But I didn't managed to make it work so far.

Here's my function code :

CREATE FUNCTION public.calcul_impact() RETURNS opaque AS '

DECLARE
id_line integer;
quantity integer;
single_price real;
total_cost real;
amort integer;
month integer;
impact real;

BEGIN

SELECT INTO id_line id_line_table FROM table WHERE id_line_table =
NEW.id_line;
SELECT INTO single_price single_price_previ FROM table WHERE id_line_table
= NEW.id_line;
SELECT INTO total_cost total_cost_previ FROM table WHERE id_line_table =
NEW.id_line;
SELECT INTO quantity quantity_previ FROM table WHERE id_line_table =
NEW.id_line;
SELECT INTO amort amortis FROM table WHERE id_line_table = NEW.id_line;
SELECT INTO month month_previ FROM table WHERE id_line_table =
NEW.id_line;
SELECT INTO impact impact_previ FROM table WHERE id_line_table =
NEW.id_line;

IF(quantity IS NULL OR single_price IS NULL) THEN impact:= 0;
ELSE IF(quantity >= 12) THEN impact:= (total_cost / amort);
ELSE IF(quantity < 12 AND single_price <= 500) THEN impact:=
total_cost;
ELSE IF(quantity < 12 AND single_price > 500) THEN impact:= ((12
- month)*(total_cost/(amort*12)));
END IF;
END IF;
END IF;
END IF;

IF (TG_OP =''INSERT'' OR TG_OP=''UPDATE'') THEN
UPDATE table SET impact_previ = impact WHERE id_line_table =
NEW.id_line;
END IF;

RETURN NEW;

END;

' LANGUAGE 'plpgsql';

CREATE TRIGGER add_impact_previ BEFORE INSERT OR UPDATE ON public.budget
FOR EACH ROW EXECUTE PROCEDURE calcul_impact();

I always get the error :

Error SQL :
ERROR: record "new" has no field named "id_ligne"

Has anyone an idea about what's wrong ?

thanks for answering me