Fix torrent hash generation to work in mixed stacks

data/fields.py

@@ -10,6 +10,9 @@ class _ResumableSHAField(TextField):
     raise NotImplementedError
 
   def db_value(self, value):
+    if value is None:
+      return None
+
     sha_state = value.state()
 
     # One of the fields is a byte string, let's base64 encode it to make sure
@@ -19,14 +22,14 @@ class _ResumableSHAField(TextField):
     return json.dumps(sha_state)
 
   def python_value(self, value):
-    to_resume = self._create_sha()
     if value is None:
-      return to_resume
+      return None
 
     sha_state = json.loads(value)
 
     # We need to base64 decode the data bytestring.
     sha_state[3] = base64.b64decode(sha_state[3])
+    to_resume = self._create_sha()
     to_resume.set_state(sha_state)
     return to_resume