groovy - How to fix "identifier of an instance of X was altered from 2 to null" in grails -

i have 2 domain classes in grails. 1 called book , other called author. book class looks this,

class book {    string name     static constraints = {       name nullable:false    } } 

and author class:

class author {    string name    book book1    book book2    book book3     static constraints = {       name nullable:false       book1 nullable:false       book2 nullable:true       book3 nullable:true    } } 

ff create author 2 books , edit 1 book got following error:

identifier of instance of com.apps.book altered 2 null 

how fix it? using grails 2.1.1 , sql server 2008.

try set null book2 , book3 properties before line authorinstance.properties = params in update method this:

book2 = null book3 = null  authorinstance.properties = params