If you have a class Book which contains a list of authors and would like to test this functionality in your Controller’s unit tests. This is how you would do it, using the Grails mockParams.

void testSaveOfBookWithAuthor() {
mockRequest.method = ‘POST’
mockParams.title = “Book Title”
mockParams[“authors”] = [[“author.id”:”123″, “author”:[“id”:”123″]]]
controller.save()
}