Fixes problem with overwriting SC fields by update

auslib/db.py

@@ -1215,6 +1215,10 @@ def mergeUpdate(self, old_row, what, changed_by, transaction=None):
         (meaning: the scheduled change and the new version of the row modify
         the same columns), an UpdateMergeError is raised."""
 
+        # Filter the update to only include fields that are different than
+        # what's in the base (old_row).
+        what = {k: v for k, v in what.items() if v != old_row.get(k)}
+
         # pyflakes thinks this should be "is False", but that's not how SQLAlchemy
         # works, so we need to shut it up.
         # http://stackoverflow.com/questions/18998010/flake8-complains-on-boolean-comparison-in-filter-clause