如何修复圣杯中的"identifier of an instance of X was altered from 2 to null"

Question

我在grails中有两个域类。一个叫书，另一个叫作者。，我的图书课是这样的

class Book {
   String name
   static constraints = {
      name nullable:false
   }
}

我的作者类：

class Author {
   String name
   Book book1
   Book book2
   Book book3
   static constraints = {
      name nullable:false
      book1 nullable:false
      book2 nullable:true
      book3 nullable:true
   }
}

如果我用两本书创建一个作者，然后将其编辑为一本书，我会出现以下错误：

identifier of an instance of com.apps.Book was altered from 2 to null

如何修复？我使用的是grails 2.1.1和sql server 2008。

Answer 1

在更新方法中，尝试在authorInstance.properties = params行之前将book2和book3属性设置为null，如下所示：

book2 = null
book3 = null
authorInstance.properties = params

Answer 2

尝试在Author类：中使用"hasOne"

class Author {
   String name
   Book book1, book2, book3
   static hasOne = [ book1:Book, book2:Book, book3:Book ]
}