Odoo 8 : Problem with fnct_inv in fields.function() not triggered - odoo

Question

I created a function fields that has this declaration :
'purchase_id': fields.function(_get_purchase_id, fnct_inv=_set_purchase_id, method=True, type="many2one", relation="purchase.order", string="Bon de commande"),
The problem is that the fnct_inv is not triggered and the field purchase_id remains empty.
Here is the definition of the method _set_purchase_id :
def _set_purchase_id(self, cr, uid, ids, field_name, field_value, arg, context):
query = """
SELECT picking_id, po.id FROM stock_picking p, stock_move m, purchase_order_line pol, purchase_order po
WHERE p.id in %s and p.id = m.picking_id and m.purchase_line_id = pol.id and pol.order_id = po.id
GROUP BY picking_id, po.id
"""
cr.execute(query, (tuple(ids), ))
picks = cr.fetchall()
_logger.info("============================================ %s " % ids)
for pick_id, po_id in picks:
self.write(cr, uid, [pick_id], {'purchase_id': [(4, po_id)]}, context=context)
I someone can help me? Or is there another solution? Thank you

Answer 1

The purpose of "inverse" methods in Odoo is not to set the field it is defined on, but to set all necessary fields for the computation of that field, so that the computation will compute the value a user was giving Odoo for that computed field.
Let me give you an example, because i don't want to work on a ready solution for your problem.
If you have a computed field days_since_last_action on a model which is computed by a date field (date_last_action) on the same model, the inverse method won't set days_since_last_action. Instead you have to take the new days_since_last_action value and reverse it to a new date.
That means for
compute method is days_since_last_action = today - date_last_action
inverse method is date_last_action = today - new value of days_since_last_action

Answer 2

In odoo8 we use Computed fields.
purchase_id = fields.Many2one('purchase.order','Bon de commande',compute='_set_purchase_id')
def _set_purchase_id(self, cr, uid, ids, field_name, field_value, arg, context):
query = """
SELECT picking_id, po.id FROM stock_picking p, stock_move m, purchase_order_line pol, purchase_order po
WHERE p.id in %s and p.id = m.picking_id and m.purchase_line_id = pol.id and pol.order_id = po.id
GROUP BY picking_id, po.id
"""
cr.execute(query, (tuple(ids), ))
picks = cr.fetchall()
_logger.info("============================================ %s " % ids)
for pick_id, po_id in picks:
self.write(cr, uid, [pick_id], {'purchase_id': [(4, po_id)]}, context=context)
For more you can refer : https://www.odoo.com/documentation/8.0/howtos/backend.html#computed-fields-and-default-values