code: def load_fields(self, *fields):
          '''Load extra fields to this :class:`StdModel`.'''
          if self._loadedfields is not None:
              if self.session is None:
                  raise SessionNotAvailable('No session available')
              meta = self._meta
              kwargs = {meta.pkname(): self.pkvalue()}
              obj = session.query(self).load_only(fields).get(**kwargs)
              for name in fields:
                  field = meta.dfields.get(name)
                  if field is not None:
                      setattr(self, field.attname,
                              getattr(obj, field.attname, None)f load_fields(self, *fields):
          '''Load extra fields to this :class:`StdModel`.'''
          if self._loadedfields is not None:
              if self.session is None:
                  raise SessionNotAvailable('No session available')
              meta = self._meta
              kwargs = {meta.pkname(): self.pkvalue()}
              obj = session.query(self).load_only(fields).get(**kwargs)
              for name in fields:
                  field = meta.dfields.get(name)
                  if field is not None:
                      setattr(self, field.attname,
                              getattr(obj, field.attname, None))
docs: Load extra fields to this :class:`StdModel`.