Queries were introduced in Chapter 5, Understanding KIE Sessions, as a way to retrieve information from a KIE Session. But queries are much more powerful than that in Drools. As a matter of fact, queries are the way Drools implements what is called backward-chaining reasoning. But before entering this new topic, and given that we were already talking about PHREAK, let's see how a regular query looks in the PHREAK network.

For this section of the book, we are going to introduce a new Java class that will be used to establish a whole-part relationship between Item objects. What this means it that an Item can now be composed of other Items:

The whole-part relationship between Items is modeled as a generic class, called IsPartOf. This generic class allows us to define non-intrusive relationships, not just between Items, but also between any other types of object in our model. As an example, if we want to specify the relation between a car, an engine, and a distributor...