Fix and test for #645

Yuck, pickle is weird.  Closes #645.

rest_framework/serializers.py

@@ -28,20 +28,23 @@ class DictWithMetadata(dict):
     def __getstate__(self):
         """
         Used by pickle (e.g., caching).
-        Overriden to remove metadata from the dict, since it shouldn't be pickled
-        and may in some instances be unpickleable.
+        Overriden to remove the metadata from the dict, since it shouldn't be
+        pickled and may in some instances be unpickleable.
         """
-        # return an instance of the first dict in MRO that isn't a DictWithMetadata
-        for base in self.__class__.__mro__:
-            if not issubclass(base, DictWithMetadata) and issubclass(base, dict):
-                return base(self)
+        return dict(self)
 
 
-class SortedDictWithMetadata(SortedDict, DictWithMetadata):
+class SortedDictWithMetadata(SortedDict):
     """
     A sorted dict-like object, that can have additional properties attached.
     """
-    pass
+    def __getstate__(self):
+        """
+        Used by pickle (e.g., caching).
+        Overriden to remove the metadata from the dict, since it shouldn't be
+        pickle and may in some instances be unpickleable.
+        """
+        return SortedDict(self).__dict__
 
 
 def _is_protected_type(obj):

rest_framework/tests/serializer.py

@@ -951,15 +951,21 @@ class SerializerPickleTests(TestCase):
             class Meta:
                 model = Person
                 fields = ('name', 'age')
-        pickle.dumps(InnerPersonSerializer(Person(name="Noah", age=950)).data)
+        pickle.dumps(InnerPersonSerializer(Person(name="Noah", age=950)).data, 0)
 
     def test_getstate_method_should_not_return_none(self):
         """
-        Regression test for
-        https://github.com/tomchristie/django-rest-framework/issues/645
+        Regression test for #645.
         """
-        d = serializers.DictWithMetadata({1: 1})
-        self.assertEqual(d.__getstate__(), serializers.SortedDict({1: 1}))
+        data = serializers.DictWithMetadata({1: 1})
+        self.assertEqual(data.__getstate__(), serializers.SortedDict({1: 1}))
+
+    def test_serializer_data_is_pickleable(self):
+        """
+        Another regression test for #645.
+        """
+        data = serializers.SortedDictWithMetadata({1: 1})
+        repr(pickle.loads(pickle.dumps(data, 0)))
 
 
 class DepthTest(TestCase):