Steps to reproduce:
1) Create a fresh grails 2.0.RC1 application
2) Create a domain class:
3) Add a property to the domain class:
4) Create a controller:
5) Add the following code to the generated index action.
5) Run the application and navigate to this URL in your browser: http://localhost:8080/grails2-test-app/book/index
As you can see on your browser, the exception is thrown only in the call to method isAttached. Other domain class dynamic methods like isDirty does not throw exception. According to grails 2.0.0RC1 http://grails.org/doc/2.0.0.RC1/ref/Domain%20Classes/isAttached.html the method is supported in this version.