Hi Prajith,

You have to override product_id_change() in the sale.order.line and update the value for the description field.

Please check the code:

class SaleOrderLine(models.Model):
    _inherit = 'sale.order.line'


    @api.multi
    @api.onchange('product_id')
    def product_id_change(self):
        res = super(SaleOrderLine, self).product_id_change()
        vals = {}
        if self.product_id.default_code:
            vals['name'] = self.product_id.name +" ["+ self.product_id.default_code +"]"
        self.update(vals)
        return res

Hope this helps.