title: How to detect which field has been updated in afterUpdate|beforeUpdate GORM methods
body: <p>i use afterUpdate method reflected by GORM API in grails project. </p>
  
  <pre><code>class Transaction{
  
      Person receiver;
      Person sender;
  
  
  }
  </code></pre>
  
  <p>i want to know which field modified to make <code>afterUpdate</code> behaves   accordingly : </p>
  
  <pre><code>class Transaction{
       //...............
     def afterUpdate(){
        if(/*Receiver is changed*/){
          new TransactionHistory(proKey:'receiver',propName:this.receiver).save();
        }
        else
        {
        new TransactionHistory(proKey:'sender',propName:this.sender).save();
        }
  
     }
  }
  </code></pre>
  
  <p>I can use <code>beforeUpdate</code>: and catch up the object before updating  in global variable (previous as Transaction), then in afterUpdate, compare <code>previous</code> with the current object. 
  Could be?</p>