base_exception: Remove side effect from api.constrains

In the documentation.

The method called by '_check_exception' has a side effect, it writes
on 'exception.rule' + on the Many2many relation between it and
the related model (such as sale.order). When decorated by
@api.constrains, any error during the method will be caught and
re-raised as "ValidationError".  This part of code is very prone to
concurrent updates as 2 sales having the same exception will both write
on the same 'exception.rule'.  A concurrent update (OperationalError) is
re-raised as ValidationError, and then is not retried properly.

Calling the same method in create/write has the same effect than
@api.constrains without shadowing the exception type.

Full explanation:
OCA#1642

base_exception/models/base_exception.py

@@ -142,14 +142,40 @@ def _get_popup_action(self):
  @api.multi
  def _check_exception(self):
  """
- This method must be used in a constraint that must be created in the
- object that inherits for base.exception.
- for sale :
- @api.constrains('ignore_exception',)
+ This method must be called in the create and write methods of the model
+ using exceptions. The model has to inherit from 'base.exception'.
+
+ Example for sale (used in OCA/sale-workflow/sale_exception):
+
+ def _fields_trigger_check_exception(self):
+ return ['ignore_exception', 'order_line', 'state']
+
+ @api.model
+ def create(self, vals):
+ record = super(SaleOrder, self).create(vals)
+ check_exceptions = any(
+ field in vals for field
+ in self._fields_trigger_check_exception()
+ )
+ if check_exceptions:
+ record.sale_check_exception()
+ return record
+
+ @api.multi
+ def write(self, vals):
+ result = super(SaleOrder, self).write(vals)
+ check_exceptions = any(
+ field in vals for field
+ in self._fields_trigger_check_exception()
+ )
+ if check_exceptions:
+ self.sale_check_exception()
+ return result
+
  def sale_check_exception(self):
- ...
- ...
- self._check_exception
+ orders = self.filtered(lambda s: s.state == 'sale')
+ if orders:
+  orders._check_exception()
  """
  exception_ids = self.detect_exceptions()
  if exception_ids: