PREV operator

The PREV operator creates a property using a previous value operator.

Syntax

PREV(propExpr)

Description

The PREV operator creates a property that returns the value of the given expression at the start of the current change session — i.e., the value that existed before the changes made in this session. It always uses the session-start scope (it does not switch modes based on the surrounding context); see the paradigm article and Events.md#change for how this relates to event-mode behavior of change operators. It is a context-dependent property operator and can appear inside expressions.

info

It's important to understand that PREV is not a built-in property with composition but an operator. Thus, in particular PREV(f(a)) is not equal to [PREV(a)](f(a)).

Parameters

• propExpr

  The expression whose previous value is returned.

Examples

CLASS A;
f = DATA INTEGER (A);
// outputs all changes f(a) in the session one by one
messageFChanges  {
    FOR CHANGED(f(A a)) DO
        MESSAGE 'In this session f(a) changed from ' + PREV(f(a)) + ' TO ' + f(a);
}

CLASS Document;
date = DATA DATE (Document);

CLASS Article;
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
setPrice  {
    price(Document d, Article a) <- PREV((GROUP LAST price(d, a) ORDER date(d), d));
}