fix QuerySet.annotate() crash with DurationField & Decimal computation

Author: timgraham

django_mongodb/features.py

@@ -102,9 +102,6 @@ class DatabaseFeatures(BaseDatabaseFeatures):
         # pymongo.errors.OperationFailure:  $multiply only supports numeric
         # types, not date. (should be wrapped in DatabaseError).
         "expressions.tests.FTimeDeltaTests.test_invalid_operator",
-        # crash in DatabaseOperations.convert_durationfield_value():
-        # unsupported type for timedelta milliseconds component: Decimal128
-        "expressions.tests.FTimeDeltaTests.test_durationfield_multiply_divide",
         # 'Ref' object has no attribute 'as_mql'.
         "expressions.tests.BasicExpressionsTests.test_aggregate_subquery_annotation",
         # AttributeError: pattern_ops

django_mongodb/operations.py

@@ -109,7 +109,12 @@ def convert_decimalfield_value(self, value, expression, connection):
 
     def convert_durationfield_value(self, value, expression, connection):
         if value is not None:
-            value = datetime.timedelta(milliseconds=value)
+            try:
+                value = datetime.timedelta(milliseconds=value)
+            except TypeError:
+                # `value` could be Decimal128 if doing a computation with
+                # DurationField and Decimal128.
+                value = datetime.timedelta(milliseconds=int(str(value)))
         return value
 
     def convert_jsonfield_value(self, value, expression, connection):