modifiedfield() method on custom table

///
///// 
public void modifiedField(FieldId _fieldId)
  {
     KWSTrainers kwsTrainers;

     super(_fieldId); 
     switch(_fieldId)
     {
       case fieldNum(KWSTrainers,TrainerId):
          select kwsTrainers where kwsTrainers.TrainerId == this.TrainerId;
          this.TrainerName = kwsTrainers.Name;
          break;
     }
  }