i have to print report using click on button.

i have one button print packing list in sales order screen.

i want to print data from other table sales.packing this is a custom module.

i have two .py files

first file new table is generate sales,packing 

sales.packing has 4 column. out of 4 column 1 is Many2one field from sale.order.

2nd is inherited module 

sales order there is a one filed one2Many to  connect sales.packing with sales order.

now,

i am clicking on packing list button. button is available in sales order form header.

but, print not showing any data for particular sales order.