Cascading validation

This has confused me for a while. If we have these classes:

class Lolrus {
    String name
    Bukkit bukkit
}

class Bukkit {
    String contents
    static constraints = {
        contents(validator: {
            return 'o.noes'
        })
    }
}

You will find this test fails because the validate() call does not cascade to the aggregated object:

void testErrorsOnAssociations() {
    def lolrus = new Lolrus(name: 'jerome')
    lolrus.bukkit = new Bukkit(contents: 'fish')
    assert !lolrus.validate()
}

The trick is validate will only cascade in the same circumstances that a save would cascade, so if we add static belongsTo = [lolrus: Lolrus] to the Bukkit class the validation cascades properly and the test works.