有关于ORM的一个问题

Post.groovy应该这样写：

package forum
class Post {
    static constraints = {
        author()
        title()
        content()
        dateCreated()
        lastUpdate()
    }
    static belongsTo = [thread: Thread]
     
    String author
    String content
    String title
    Date dateCreated
    Date lastUpdate
}

Thread.groovy应该这样写：

package forum
class Thread {
    static constraints = {
        lastUpdate()
    }
    static hasMany = [posts: Post]
    Date lastUpdate
}