[GRAILS]how to use the String type for the primary key column

How to use the String type primary key column in Domain Class on Grails environment.

• the definition of the Domain Class
  

  class Test {
        //define id column as String type
        String id
        .....
        //define the id generator type, it should use the manual assignment
        static mapping={
                id generator:'assigned'
        }
  }
  

• using this domain class
  

          def index = {
                 //create new object defined the primary key as the String type.
                 def test=new Test()
  
                 def date=new Date()
                 //set the unique value of primary key manually.
                 test.setId("pk-${date.getTime()}")
                 test.setMsg("Message")
                 //save
                 test.save()
                 ...
         }