Hey so I've tried the following code :

# Free unit
@api.onchange('free_unit')
def onchange_free_unit(self):
    if self.free_unit > 0:
        # looking for the id
        search_ids = self.env['purchase.order'].search([])
        last_id = search_ids and max(search_ids)
        print last_id.id + 1
        print self._origin.order_id.id

        # check if the order line is saved
        if last_id.id == self._origin.order_id.id:
            print 'same !!!! \n'
            new_line = {
                'order_id': self._origin.order_id.id,
                'product_id': self.product_id.id,
                'name': self.name,
                'product_qty': self.product_qty,
                'date_planned': self.date_planned,
                'date_ok': self.date_ok,
                'product_uom': 1,
                'price_unit': self.price_unit,
                'taxes_id': self.taxes_id,
                'cascade_bool': True,
            }
            print "new_line", new_line
            self.create(new_line)
        if not self._origin.order_id.id:
            print 'not same !!! \n'
            new_line = {
                'order_id': last_id.id + 1,
                'product_id': self.product_id.id,
                'name': self.name,
                'product_qty': self.product_qty,
                'date_planned': self.date_planned,
                'date_ok': self.date_ok,
                'product_uom': 1,
                'price_unit': self.price_unit,
                'taxes_id': self.taxes_id,
                'cascade_bool': True,
            }
            print "new_line", new_line
            self.create(new_line)

So I'm trying to retreive the order_id to put it in the object. But it seems like it's impossible to do it in draft mode. I mean when the object is not saved.

I can do it when the object is saved and edit mode, but not in my purchase order before.

Any ideas ?? Or is it impossible ?

Thanks