diff --git a/addons/mail/models/mail_activity.py b/addons/mail/models/mail_activity.py
index 5e5741cf6e6..4d2795cbe75 100644
--- a/addons/mail/models/mail_activity.py
+++ b/addons/mail/models/mail_activity.py
@@ -635,6 +635,28 @@ class MailActivity(models.Model):
     # TOOLS
     # ----------------------------------------------------------------------
 
+    def _classify_by_model(self):
+        """ To ease batch computation of various activities related methods they
+        are classified by model. Activities not linked to a valid record through
+        res_model / res_id are ignored.
+
+        :return dict: for each model having at least one activity in self, have
+          a sub-dict containing
+            * activities: activities related to that model;
+            * record IDs: record linked to the activities of that model, in same
+              order;
+        """
+        data_by_model = {}
+        for activity in self.filtered(lambda act: act.res_model and act.res_id):
+            if activity.res_model not in data_by_model:
+                data_by_model[activity.res_model] = {
+                    'activities': self.env['mail.activity'],
+                    'record_ids': [],
+                }
+            data_by_model[activity.res_model]['activities'] += activity
+            data_by_model[activity.res_model]['record_ids'].append(activity.res_id)
+        return data_by_model
+
     def _prepare_next_activity_values(self):
         """ Prepare the next activity values based on the current activity record and applies _onchange methods
         :returns a dict of values for the new activity
diff --git a/addons/mail/models/mail_thread.py b/addons/mail/models/mail_thread.py
index 43984c8b2df..5e21146141d 100644
--- a/addons/mail/models/mail_thread.py
+++ b/addons/mail/models/mail_thread.py
@@ -470,7 +470,7 @@ class MailThread(models.AbstractModel):
         """
         partner_fields = self._mail_get_partner_fields()
         return dict(
-            (record.id, self.env['res.partner'].union(*[self[fname] for fname in partner_fields]))
+            (record.id, self.env['res.partner'].union(*[record[fname] for fname in partner_fields]))
             for record in self
         )