The previous value operator creates a property that returns the value of the specified property at the beginning of the session (that is, the current value in the database ignoring the session changes).
This operator is calculated differently inside the event handling: here it returns the value at the time of the previous occurrence of this event (or rather, at the time when all its handling were completed).
To declare a property that returns a previous value, use the
f = DATA INTEGER (A);
// outputs all changes f(a) in the session one by one
FOR CHANGED(f(A a)) DO
MESSAGE 'In this session f(a) changed from ' + PREV(f(a)) + ' TO ' + f(a);
date = DATA DATE (Document);
price = DATA NUMERIC[14,2] (Document, Article);
// write in the price of the document the last used price in the database
// PREV is important to ignore the prices entered in this document
// this is especially important if the last used price is materialized, then the platform can simply read this value from the table
price(Document d, Article a) <- PREV((GROUP LAST price(d, a) ORDER date(d), d));