hi 
I have a new module . 
on this module i want to create a backorder from a picking .
so my class is like that : 

class PickingProcess(models.Model):
    _inherit = "mail.thread"
    _name = 'picking.process'
    _description = "picking process ecoparc"
    @api.multi
    def createbackorder(self, picking_id):
       picking = self.env['stock.picking'].browse(picking_id)
       for pack in picking.pack_operation_ids:
           pack.write({'qty_done': 1})
       backorder = self.env['stock.backorder.confirmation'].create({'pick_id': picking.id})
       backorder.process()

     @api.model
     def manage_action(self):
       self.env['picking.process'].createbackorder(MY_PICKING_ID)
       #self.createbackorder(MY_PICKING_ID) do the same ....

    @api.onchange('input_action')
    def on_change_input_action(self):
        if self.input_action == "" :
           return 
        self.manage_action()    

when i change my input it calls the method on_change_input_action which call manage_action. In this case the backorder doesn t work correctly . It makes as done the picking with qty 1 but it doesn t create the backorder.

BUT if i call createbackorder() from an other class like that self.env['picking.process'].createbackorder(MY_MPICKING_ID), it works and create correctly the backorder.
Could you explain to me why ? i spent 2 days to understand but i m lost and i didn t find the answer!

Thanks a lot by advance 

Thomas