Previous value (PREV)

The previous value operator creates a property that returns the value of the given expression at the beginning of the change session (that is, the current value in the database, ignoring any changes that have been made in the session). Together with the change operators — which are derived from it — this operator is what makes session-local change tracking possible: it gives access to the "before" state that the current session is being compared against.

info

This operator always uses the start-of-session scope. For how that interacts with event handling, see the canonical event-mode section.

Language

To declare a property that returns a previous value, use the PREV operator.

Examples

CLASS Order;
sum = DATA NUMERIC[10,2] (Order);

// Most common use: read the "before" value inside an event handler reacting to a
// change. PREV(sum(o)) returns the value at the start of the session — the database
// value, ignoring any in-session change. Change predicates (CHANGED, SET, DROPPED,
// ...) are derived from PREV as Boolean comparisons against the current value; PREV
// itself gives access to the actual previous value.
WHEN CHANGED(sum(Order o)) DO
    MESSAGE 'Sum changed: ' + PREV(sum(o)) + ' → ' + sum(o);