code: def _serialize(self):
          
          result = { a: getattr(self, a) for a in type(self).properties
              if type(self).properties[a].mutable }
  
          for k, v in result.items():
              if isinstance(v, Base):
                  result[k] = v.id
  
          return result
docs: A helper method to build a dict of all mutable Properties of
          this object