public RelationshipBuilder addColumns(String fromCol,String toCol){
  _fromCols.add(fromCol);
  _toCols.add(toCol);
  return this;
}

public RelationshipBuilder addColumns ( String fromCol , String toCol ) { _fromCols . add ( fromCol ) ; _toCols . add ( toCol ) ; return this ; }
adds a pair of columns to the relationship .
