CategoryController.groovy

package com.odobo.gr8conf

import grails.rest.RestfulController
import grails.transaction.Transactional

@Transactional(readOnly = true)
class CategoryController extends RestfulController {

    static responseFormats = ['json', 'xml']

    CategoryController() {
        super(Category)
    }


}

CategorySpec.groovy

class CategorySpec extends Specification {

    @Shared
    RESTClient restClient = new RESTClient("http://localhost:8080/restful-gr8conf-2014")

    def "it returns just the categories of a game when is a nested resource"() {
        when:
        Response response = restClient.get(path: "/games/5/categories", accept: ContentType.JSON)
        println response.json

        then:
        response.json.size() == 2
    }

}

GameController.groovy

package com.odobo.gr8conf

import grails.rest.RestfulController
import grails.transaction.Transactional

@Transactional(readOnly = true)
class GameController extends RestfulController {

    static responseFormats = ['json', 'xml']

    GameController() {
        super(Game)
    }

}