How to refactor in Grails

I'm doing a project that requires some refactoring.

My domain class is like:

Book {

static belongsTo[category: category]
Category category;
String name;

}

Category {

static hasMany [books: Book]
String name;
Category parent;

}

Now I want to change that one book may belongsTo many categories. I know how to transform the domains, but overwhelmed by the fact that I must change every single appearance of category in my view/controller/service.

For example, when a book can be in 开发者_JAVA百科2 categories, I no longer can use the navigator like "Sciences > Math > Math for elemental class".

I'm new in this project, so that I also fear that I may broke something while trying to repair the code.

Did anyone have the same experience? Is there any tip that can help reduce the complexity of this refactoring work?

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Intellij Idea is very good at refactoring Java and is good enough with Grails.

Still, you'd better covered the changed classes with unit tests.

You can also provide a fallback transient property getCategory() { categories.size() == 1 ? categories[0] : null } - this will hide some failures, and remove it only after you clean up with other problems introduced.

Sorry, nothing besides common advice.