Brief: constraints

Constraints (CONSTRAINT)

A constraint is a property whose value must always be NULL. It is checked on the event given by the event description block of the statement — by default, the global APPLY event; if by that moment it has become not NULL on at least one object set, the platform shows a message listing those sets and cancels the changes. It is created by the CONSTRAINT statement:

CONSTRAINT [eventClause] constraintExpr [CHECKED [BY propertyId1, ..., propertyIdN]] MESSAGE messageExpr
    [PROPERTIES outExpr1, ..., outExprM];

The CHECKED BY option makes the dialog that changes the listed properties filter out the values that would violate the constraint.

Simple constraints are set by the kind of relation between properties rather than by an expression, Two kinds are implemented: consequence — the => statement — and definiteness — the NONULL option. The consequence can resolve a violation itself: its RESOLVE [LEFT] [RIGHT] clause tells the platform which side to correct. NONULL has no such clause — it takes [DELETE], which deletes the offending objects instead.

Analogy: CHECK and NOT NULL in SQL, except that the condition is an arbitrary property over the whole database.

CONSTRAINT balance(Sku s, Stock st) < 0 MESSAGE 'Balance cannot be negative';

Aggregated objects and constraints

An aggregation creates an aggregated object when the aggregated property becomes not NULL on a combination of parameter values, and deletes that object once the property becomes NULL again. Only the AGGR operator creates and deletes the object; GROUP AGGR builds the reverse property — from the parameter values back to the object. Both are described in Brief: properties.

For integrity this is one more invariant the platform maintains on its own: at most one aggregated object per combination of parameter values. GROUP AGGR adds it as a constraint over its group, and AGGR keeps the correspondence between the objects and the parameter values within it. How that constraint is maintained is described in aggregations and simple constraints.

// a shipment is created for an invoice when the create-shipment flag is set, and deleted when it is dropped
shipment (Invoice i) = AGGR ShipmentInvoice WHERE createShipment(i);