def ids(self):
        """Convenience method to get the ids for Materials present"""
        assert self.name == "Materials"
        ids = list()
        for attr in self.attrs:
            attr_obj = getattr(self, attr)
            if hasattr(attr_obj, 'Id'):
                ids.append(getattr(attr_obj, 'Id'))
        return ids
Convenience method to get the ids for Materials present