How to Analyze Multiple Objects of the Same Type in a Business Rule

In many scenarios, you will write a business rule against a type and expect each instance of the type that is asserted into the engine to be separately analyzed and acted upon by the rule. In some scenarios, however, you will want to analyze multiple instances of a given type simultaneously in a rule.

Take for example a rule that uses instances of the FamilyMember class.

IF FamilyMember.Role == Father  
AND FamilyMember.Role == Son  
AND FamilyMember.Surname == FamilyMember.Surname  
THEN FamilyMember.AddChild(FamilyMember)  

The rule identifies a FamilyMember instance that is a Father and another instance that is a Son. If the instances are related by surname, the Son instance is added to a collection of children on the Father instance. If each FamilyMember instance were analyzed separately in the rule, the rule would never fire, because in this scenario, the FamilyMember only has one role—Father or Son.

Therefore, you must indicate to the engine that multiple instances should be analyzed together in the rule, and you need a way to differentiate the identity of each instance in the rule. The Instance ID property is used to provide this functionality. This field is available in the Properties window when a fact is selected in Facts Explorer. You should change the value of the field prior to dragging a fact or member into a rule.

Using the Instance ID property, the rule would be rebuilt. For those rule arguments that use the Son instance of FamilyMember, the Instance ID field is changed from the default of 0 to 1. When the Instance ID is changed from 0 and the fact or member is dragged into the rule editor, the value of Instance ID will show up in the rule after the class.

IF FamilyMember.Role == Father  
AND FamilyMember(1).Role== Son  
AND FamilyMember.Surname == FamilyMember(1).Surname  
THEN FamilyMember.AddChild(FamilyMember(1))  

Now, assume that a Father instance and a Son instance are asserted into the engine. The engine will evaluate the rule against the various combinations of the instances. Assuming that the Father and Son instance have the same surname, the Son instance will be added to the Father instance as intended.