Self-heal statistics results model

Happens when multiple results are created using the
same slug value. Forum topic 1404.

Signed-off-by: Roberto Rosario <roberto.rosario@mayan-edms.com>

HISTORY.rst

@@ -148,6 +148,9 @@
 - Support running specific tests inside the Docker container.
   docker run --rm mayanedms/mayanedms:3.3 run_tests
 - Make the statistics slug field unique.
+- Self-heal statistics results model when multiple
+  results are created using the same slug value.
+  Forum topic 1404.
 
 3.2.10 (2019-XX-XX)
 ===================

docs/releases/3.3.rst

@@ -211,6 +211,7 @@ Backward incompatible changes
 Bugs fixed or issues closed
 ---------------------------
 
+- :forum-topic:`1404` Mayan EDMS 3.2.8 statistics error
 - :gitlab-issue:`399` Archive of processed e-mails
 - :gitlab-issue:`526` RuntimeWarning: Never call result.get() within a task!
 - :gitlab-issue:`532` Workflow preview isn't updated right after transitions are modified
@@ -222,5 +223,4 @@ Bugs fixed or issues closed
 - :gitlab-issue:`640` UX: "Toast" Popup position prevents access to actions
 - :gitlab-issue:`644` Update sane-utils package in docker image.
 
-
 .. _PyPI: https://pypi.python.org/pypi/mayan-edms/

mayan/apps/mayan_statistics/classes.py

@@ -158,6 +158,11 @@ class Statistic(object):
             return StatisticResult.objects.get(slug=self.slug)
         except StatisticResult.DoesNotExist:
             return StatisticResult.objects.none()
+        except StatisticResult.MultipleObjectsReturned:
+            # This should not happen. Self-heal by deleting the duplicate
+            # results.
+            StatisticResult.objects.filter(slug=self.slug).delete()
+            return StatisticResult.objects.none()
 
     def get_results_data(self):
         results = self.get_results()