domainsAndInterfaces.groovy

interface Buyable {
    def buy()
}

class SomeBuyableObject implements Buyable {
    String foo
    def buy() { //do stuff }
}

class AnotherBuyableObject implements Buyable {
    String bar
    def buy() { //do stuff }
}

class MyService {
    def listBuyables() {
        def s = new SomeBuyableObject(foo: 'foo').save()
        def a = new AnotherBuyableObject(bar : 'bar').save()
    
        def buyables = [s, a]*.findAll().sum() // GORM find all
    
        assert buyables.size() == 2
    }
}