无需依赖删除的GORM关系

使用这样的联接表：

class User {    Set<Node> getNodes() {        UserNode.executeQuery("select x.node from UserNode x where x.user = :user", [user: this])    }    def addTonodes(Node node) {       def userNode = new UserNode()       userNode.user = this       userNode.node = node       userNode.save()          }}class Node {      Set<User> getUsers() { ... }}class UserNode {   User user   Node node}