docs: def _write_audit(self, user, message, changed_version):
          """
          Creates an audit entry, which is much like a snapshot in that
          it references versions and provides some history of the changes made.
          """
          audit = {'_id': bson.ObjectId(),
                   'user': user,
                   'message': message,
                   'symbol': changed_version.symbol
                   }
          orig_version = changed_version.orig_version.version
          new_version = changed_version.new_version.version
          audit['orig_v'] = orig_version
          audit['new_v'] = new_version
          # Update the versions to contain the audit
          mongo_retry(self._versions.update_many)({'symbol': changed_version.symbol,
                                                   'version': {'$in': [orig_version, new_version]}
                                                   },
                                                  {'$addToSet': {'parent': audit['_id']}})
          # Create the audit entry
          mongo_retry(self._audit.insert_one)(audit)