How to sum column in View Object using Groovy

Use Case : 
we have 2 tables PayOrders  and  PayOrderTrans 

 we need to :  sum Credit column in PaymentOrderTrans table

• First create New Attribute called CreditTotal 

• Go to Default Value section
• Choose Expression
• Enter the following expression

         object.getRowSet().sum('Credit')

  What about the Null value ?   

     to avoid null value, you must modify the expression to

          object.getRowSet().sum('Credit != null ? Credit : 0 ')