Why is GORM performing cascading delete, even if I omit the belongsTo keyword?
class Holiday {
    String justification
    User user
    //static belongsTo = User
        static constraints = {
        }
}

class User {
    String login
    String password
    static hasMany = [ holidays : Holiday ]
        static constraints = {
    }
}

void testWithoutBelongsTo() {
        def user1 = new User(login:"anto", password:"secret")
        user1.save()
        def holiday1 = new Holiday(justification:"went to trip")
        holiday1.save()
        user1.addToHolidays(holiday1)
        assertEquals 1, User.get(user1.id).holidays.size()
        user1.delete()
        assertFalse User.exists(user1.id)
        assertFalse Holiday.exists(holiday1.id)
    }

class Holiday {
    String justification
    User user
    //static belongsTo = User
    static constraints = {
        user(nullable: true)
    }
}

void testWithoutBelongsTo()
{
    def user1 = new User(login:"anto", password:"secret")
    user1.save(failOnError: true)
    def holiday1 = new Holiday(justification:"went to trip",
                               user: user1) // Set user properly
    holiday1.save(failOnError: true)
    user1.addToHolidays(holiday1)
    assert 1, User.get(user1.id).holidays.size()
    holiday1.user = null  // Unset user as otherwise your DB
                          // won't be happy (foreign key missing)
    user1.delete()
    assert ! User.exists(user1.id)
    assert Holiday.exists(holiday1.id)
 }