[IMP] mail: encapsulate _message_track in batch

_message_track is encapsulated in a method that calls it in batch. The purpose
is to ease the tracking to work in batch. Moreover future improvements in mail
will use this method and will be able to work in batch.

addons/mail/models/mail_thread.py

@@ -415,8 +415,21 @@ class MailThread(models.AbstractModel):
         """
         return False
 
+    @api.multi
+    def _message_track_get_changes(self, tracked_fields, initial_values):
+        """ Batch method of _message_track. """
+        result = dict()
+        for record in self:
+            result[record.id] = record._message_track(tracked_fields, initial_values[record.id])
+        return result
+
     @api.multi
     def _message_track(self, tracked_fields, initial):
+        """ For a given record, fields to check (tuple column name, column info)
+        and initial values, return a structure that is a tuple containing :
+
+         - a set of updated column names
+         - a list of changes (initial value, new value, column name, column info) """
         self.ensure_one()
         changes = set()
         tracking_value_ids = []
@@ -437,11 +450,16 @@ class MailThread(models.AbstractModel):
 
     @api.multi
     def message_track(self, tracked_fields, initial_values):
+        """ Track updated values. Comparing the initial and current values of
+        the fields given in tracked_fields, it generates a message containing
+        the updated values. This message can be linked to a mail.message.subtype
+        given by the ``_track_subtype`` method. """
         if not tracked_fields:
             return True
 
+        tracking = self._message_track_get_changes(tracked_fields, initial_values)
         for record in self:
-            changes, tracking_value_ids = record._message_track(tracked_fields, initial_values[record.id])
+            changes, tracking_value_ids = tracking[record.id]
             if not changes:
                 continue