'Lookup the :class:`Field` instance that this :class:`FieldRef`
references.'

def get_ref(self):

    for field in self._table._votable.iter_fields_and_params():
        if (isinstance(field, Field) and (field.ID == self.ref)):
            return field
    vo_raise(u"No    field    named    '{}'".format(self.ref), self._config, self._pos, KeyError)
