odoo 10 : create order from custom module - odoo-10

Question

I have a new model in a custom module .
in this module i want to do a backorder on a order (1 product with 2 of quantity)
I want to do this backorder when a field is changed on a form
When i do that i have this error :
Traceback (most recent call last): File
"/usr/lib/python2.7/dist-packages/odoo/http.py", line 642, in
_handle_exception
return super(JsonRequest, self)._handle_exception(exception) File "/usr/lib/python2.7/dist-packages/odoo/http.py", line 684, in
dispatch
result = self._call_function(**self.params) File "/usr/lib/python2.7/dist-packages/odoo/http.py", line 334, in
_call_function
return checked_call(self.db, *args, **kwargs) File "/usr/lib/python2.7/dist-packages/odoo/service/model.py", line 101, in
wrapper
return f(dbname, *args, **kwargs) File "/usr/lib/python2.7/dist-packages/odoo/http.py", line 327, in
checked_call
result = self.endpoint(*a, **kw) File "/usr/lib/python2.7/dist-packages/odoo/http.py", line 942, in call
return self.method(*args, **kw) File "/usr/lib/python2.7/dist-packages/odoo/http.py", line 507, in
response_wrap
response = f(*args, **kw) File "/usr/lib/python2.7/dist-packages/odoo/addons/web/controllers/main.py",
line 895, in call_kw
return self._call_kw(model, method, args, kwargs) File "/usr/lib/python2.7/dist-packages/odoo/addons/web/controllers/main.py",
line 887, in _call_kw
return call_kw(request.env[model], method, args, kwargs) File "/usr/lib/python2.7/dist-packages/odoo/api.py", line 689, in call_kw
return call_kw_multi(method, model, args, kwargs) File "/usr/lib/python2.7/dist-packages/odoo/api.py", line 680, in
call_kw_multi
result = method(recs, *args, **kwargs) File "/usr/lib/python2.7/dist-packages/odoo/models.py", line 5535, in
onchange
value = record[name] File "/usr/lib/python2.7/dist-packages/odoo/models.py", line 5269, in
getitem
return self._fields[key].get(self, type(self)) File "/usr/lib/python2.7/dist-packages/odoo/fields.py", line 930, in
get
value = record._cache[self] File "/usr/lib/python2.7/dist-packages/odoo/models.py", line 5620, in
getitem
value = self._recs.env.cache[field][self._recs.id] KeyError: <odoo.models.NewId object at 0x7f312b74d150>
Here is my model :
class PickingProcess(models.Model):
_inherit = "mail.thread"
_name = 'picking.process'
_description = "picking process ecoparc"
input_action = fields.Char("Action" ,store=True)
#api.one
#api.depends('input_action')
def _compute_imput_action2(self):
ctx = dict(self._context or {})
_logger.error(ctx)
pic = self.env['stock.picking'].browse(4910)
for pack in pic.pack_operation_ids:
pack.qty_done = 1
backOrderModel = self.env['stock.backorder.confirmation']
backorderObj = backOrderModel.create(
{'pick_id': 4910})
backorderObj._process(cancel_backorder=False)
return
Do you have an idea why i have this error and how to fix it please?
Thanks a lot
Thomas