Merge pull request #316 from edx/shr/bug/revert-plat-736

Revert "String field return empty string for None"

xblock/fields.py

@@ -808,19 +808,11 @@ class String(JSONField):
     MUTABLE = False
 
     def from_json(self, value):
-        if value is None:
-            return ''
-        elif isinstance(value, basestring):
+        if value is None or isinstance(value, basestring):
             return value
         else:
             raise TypeError('Value stored in a String must be None or a string, found %s' % type(value))
 
-    def to_json(self, value):
-        if value is None:
-            return ''
-        self._warn_deprecated_outside_JSONField()
-        return value
-
     def from_string(self, value):
         """String gets serialized and deserialized without quote marks."""
         return self.from_json(value)

xblock/test/test_fields.py

@@ -212,14 +212,7 @@ def test_json_equals(self):
         self.assertJSONOrSetEquals('', '')
 
     def test_none(self):
-        # test for NoneType for to_string and from_string methods
-        test_field = String(enforce_type=True)
-
-        result_to_string = test_field.to_string(None)
-        self.assertEquals(result_to_string, '')
-
-        result_from_string = test_field.from_string(None)
-        self.assertEquals(result_from_string, '')
+        self.assertJSONOrSetEquals(None, None)
 
     def test_error(self):
         self.assertJSONOrSetTypeError(['a'])