Aggregations

Aggregation is the creation of a unique (aggregate) of the object corresponding to each non-NULL value of some aggregated property. Each such object is assumed to have properties that map this object to each aggregated property parameter, and, conversely, a property that maps the aggregated property parameters to this object.

The aggregated object and each aggregated property parameter must belong to a specified class.

The aggregation mechanism is driven by three events:

• Base event — the check event: the platform verifies that for every non-NULL value of the aggregated property a corresponding aggregated object exists, and vice versa.
• Create event — the resolution event: when the aggregated property becomes non-NULL and no aggregated object exists yet, the platform creates it and fills its properties with the parameter values.
• Delete event — the resolution event: when the aggregated property becomes NULL and the aggregated object still exists, the platform deletes it.

In a particular aggregation all three events may coincide, or the create and delete events may differ both from the base event and from each other.

Language

To create aggregations, use the operatorAGGR.

Examples

CLASS A; CLASS B; CLASS C;
f = DATA INTEGER (A, B);
c = AGGR C WHERE f(A a, B b) MATERIALIZED INDEXED;

CLASS AB;
ab = AGGR AB WHERE A a IS A AND B b IS B; // for each A B pair creates an object AB

CLASS Shipment 'Delivery';
date = ABSTRACT DATE (Shipment);
CLASS Invoice 'Invoice';
createShipment 'Create delivery' = DATA BOOLEAN (Invoice);
date 'Shipment date' = DATA DATE (Invoice);
CLASS ShipmentInvoice 'Delivery by invoice' : Shipment;
// creating a delivery by invoice, if the option for delivery creation is defined for the invoice
shipment(Invoice invoice) = AGGR ShipmentInvoice WHERE createShipment(invoice); 
date(ShipmentInvoice si) += sum(date(invoice(si)),1); // delivery date = invoice date + 1