[firebird-support] What makes a column read only?

[Werner F. Bruhin]

Hi,

I have this table definition:
CREATE TABLE CONSUMPTION
(
   ID KEYS NOT NULL,
   QUANTITY Integer DEFAULT 0,
   UNITPRICE Numeric(18,2) DEFAULT 0,
   CONSUMEDVALUE Numeric(18,2) DEFAULT 0,
   REMARKS Varchar(50) COLLATE UNICODE_CI_AI,
   CONSUMED Timestamp,
   MATURITYFIRST Integer,
   MATURITYBEST Integer,
   MATURITYPAST Integer,
   TRANSFER_ID Bigint,
   FK_CBBOTTLEID Bigint,
   FK_CELLARBOOK_ID KEYS,
   FK_RATING_ID KEYS,
   FK_CELLAR_ID KEYS,
   FK_REASON_ID KEYS,
   CREATED_AT Timestamp,
   UPDATED_AT Timestamp,
   CREATED_BY KEYS,
   UPDATED_BY KEYS,
   CONSTRAINT PK_CONSUMPTION PRIMARY KEY (ID)
)

CREATE TRIGGER CONSUMPTION_CONSVAL FOR CONSUMPTION
ACTIVE AFTER INSERT OR UPDATE POSITION 0
AS
BEGIN
     /* enter trigger code here */
     new.CONSUMEDVALUE = new.UNITPRICE * new.QUANTITY;
END

When I try to update manually (e.g. using FlameRobin or IBExpert) the 
column "CONSUMEDVALUE" or when I try to create the above trigger I get 
this error:

SQL Message : -151
This column cannot be updated because it is derived from an SQL function 
or expression.

Engine Code    : 335544359
Engine Message :
attempted update of read-only column

What am I doing wrong?

Werner