Implements metamodel graph walking. In layman terms, we are walking the graph of the users domain model as defined/understood by mapped associations.

Initially grew as a part of the re-implementation of the legacy JoinWalker functionality to instead build LoadPlans. But this is really quite simple walking. Interesting events are handled by calling out to implementations of AssociationVisitationStrategy which really provide the real functionality of what we do as we walk.

The visitor will walk the entire metamodel graph (the parts reachable from the given root)!!! It is up to the provided AssociationVisitationStrategy to tell it when to stop. The walker provides the walking; the strategy provides the semantics of what happens at certain points. Its really very similar to parsers and how parsing is generally split between syntax and semantics. Walker walks the syntax (associations, identifiers, etc) and when it calls out to the strategy the strategy then decides the semantics (literally, the meaning).

The visitor will, however, stop if it sees a "duplicate" AssociationKey. In such a case, the walker would call AssociationVisitationStrategy#foundCircularAssociation and stop walking any further down that graph any further.