sencha ext js 6 modern joincolumn sort property change

Question

I want to find method named by getSortParam which is used by classic version. It is hard to find modern version, so I couldn't solve the problem. Could you give me other advice?

class version :

     bind: '{Return}',  
     columns: [
          { text: 'box_barcode',           
            dataIndex: 'boxBarcode',    
            flex: 1 
           },
          { text: 'return_seq', 
            dataIndex: 'Id',
            flex: 1,
            getSortParam: function() {
                  return 'ReturnSch.Id';
             }
          },
     ]