Grails: good tutorial/explanation for non-trivial testing

I am working on a super sweet Grails project.

I get simple tests, like having some made-up data objects to mess with validators, but what about when I have 30 different domain-classes, only 1 i开发者_开发知识库s modified by users - the rest by services in both an event-driven and periodic fashion... what can I even test for?

Is there some discussion on testing theory I should read? Am I totally missing it?

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Chapter 7 of Grails in Action is what explained it best to me.